Russia's premier warship in the Black Sea sustained severe damage and sank on its way back to port early Thursday, a major symbolic blow to Moscow as the invasion of Ukraine entered its eighth week and both sides prepare for a potentially devastating battle over the eastern Donbas region. 

The cause of the explosion on the missile cruiser Moskva remains contested, with Russia saying a fire detonated ammunition onboard and forced the crew to evacuate. Ukraine said that it struck the Russian vessel with a missile, and a senior U.S. official told The Washington Post on Thursday evening that the ship sank as a result of a Ukrainian attack but did not confirm what weapon was used.

Earlier in the day, U.S. national security adviser Jake Sullivan called the damage to the ship a setback for Russia regardless of how it was disabled. Either it was "just incompetence" or "they came under attack," he said at a breakfast. "Neither is a particularly good outcome for them."

The war in Ukraine, which President Joe Biden this week described as genocide, has killed thousands of civilians, forced more than 4.7 million Ukrainians to flee the country and reordered the geopolitical landscape in Europe.

Smaller European nations that also feel threatened by Russia have rushed weapons to Ukraine, while Finland and Sweden signaled Wednesday that they would consider joining NATO, ending their nonaligned status. On Thursday, Russia warned the two countries that if they join the alliance, Moscow will strengthen its military forces in the Baltic Sea, including though the deployment of nuclear weapons.

The Biden administration, meanwhile, has vastly expanded the types of weapons it will provide to the Ukrainian military, with the next phase of the conflict expected to include large clashes in the open fields of the Donbas region. An additional $800 million in security assistance will for the first time include anti-personnel mines, long-range artillery, armored vehicles and radar defense equipment. And the top U.S. commander in Europe and his staff are developing training for Ukrainian forces that will take place on the continent and teach the soldiers about weapons new to the country's arsenal, a senior U.S. defense official said Thursday.

The training will focus on using 155mm howitzer cannons, counter-artillery radar and Sentinel air defense radars, and will last a few days each, the official said, speaking on the condition of anonymity under ground rules set by the Pentagon.

Russia this week sent a formal diplomatic note to the United States warning that U.S. and NATO shipments of the "most sensitive" weapons systems to Ukraine was "adding fuel" to the conflict there and could bring "unpredictable consequences."

A copy of the diplomatic démarche -- the first known high-level communication to the United States from Russia since the invasion began -- was seen by The Washington Post. The State Department declined to comment on whether any response had been sent. Top Russian military officials have declined attempts by their U.S. counterparts to speak with them during the conflict.

The Russian embassy did not respond to requests for comment on Thursday.

Among the "most sensitive" items mentioned in the note are "multiple launch rocket systems" -- which Russian forces have deployed in territory they control but which are not believed to have come from the United States or its NATO allies. Russia accused the allies of violating "rigorous principles" governing the use of arms transfers to conflict zones and of being oblivious to "the threat of high-precision weapons falling into the hands of radical nationalists, extremists and bandit forces in Ukraine."

It accused NATO of trying to pressure Ukraine to "abandon" sputtering and so far unsuccessful negotiations with Russia, "in order to continue the bloodshed." Washington, it said, was pressuring countries with military and technical cooperation with Russia to "immediately curtail it," and those with Soviet-era weapons to transfer them to Ukraine.

"We call on the United States and its allies to stop the irresponsible militarization of Ukraine, which implies unpredictable consequences for regional and international security," the note said.

Sullivan said the weaponry that is headed to Ukraine represented "everything we could muster in a reasonable time frame" so it could be "ready to use for the coming battle in the east." He stressed that the U.S. was "not operating inside the territory of Ukraine" -- a red line that could provoke a response from Moscow -- and is making sure there are "resilient and diverse" ways to continue delivering military aid into the country.

Biden told reporters his administration is also considering whether to send a senior U.S. government official to Ukraine in a gesture of solidarity, following visits this week by top European leaders. "We're making that decision now," the president said.

The death toll from an April 8 Russian airstrike on a train station in Kramatorsk rose to 59 after two children injured in the attack died, Ukraine's Defense Ministry said Thursday. Seven of the fatalities were children, the ministry said.

"A bloody children's toy will be sent by [Ukraine's national police] to the U.N. as proof of this barbaric crime," the ministry said in a post on Twitter, including an image of a blood-soaked toy horse apparently left at the scene.

Another turning point in the war may be imminent in the embattled port city of Mariupol. The Russian Defense Ministry said 134 Ukrainian soldiers had surrendered there overnight. A day earlier, the ministry asserted that Mariupol was "completely liberated" from the Azov Battalion -- a Ukrainian paramilitary unit with a history of far-right nationalism -- and said the remaining Ukrainian forces in the city were trapped.

But a local Ukrainian official said Thursday on Telegram that Ukrainian and Russian forces continue to clash, particularly near the harbor. The city's mayor, Vadym Boychenko, recently estimated that more than 10,000 civilians have died in the Russian siege.

In its daily assessment for April 13, the Institute for the Study of War reported that Russian forces could "capture Mariupol in the coming week," adding that the city's defenders wouldn't be able to hold out indefinitely.

Russian forces, it reported, continued to take ground in the city -- including driving Ukrainian troops to abandon a metal plant in the north, "further constricting the two remaining pockets of Ukrainian defenders."

Ukrainian officials have described Mariupol as a crucial battlefield. "Mariupol is the heart of this war today," Ukrainian President Volodymyr Zelenskyy told the Associated Press on Sunday. "It beats, we fight, we are strong. If it stops beating, we will have weaker positions."

Austrian Chancellor Karl Nehammer, who recently held meetings just days apart with Russian President Vladimir Putin and Zelenskyy, told Germany's DPA news agency that the two leaders are entrenched in a "wartime state of mind."

"Both sides are preparing for a very intense and, from a humane perspective, devastating battle" in the Donbas region, he said.

The sinking of the Moskva warship will be a major morale boost for Ukrainian forces, experts said. Late Thursday, the Russian defense ministry said that the Moskva sank as it was being towed to port. Pentagon spokesman John Kirby said the impact of the loss of the ship on the broader war effort was not immediately clear. "The naval component of the war has been fairly limited to two things: One, cruise missile strikes into Ukraine and, two, replenishment and resupply of their efforts in the south," Kirby said.

The Pentagon also disclosed Thursday for the first time that a small group of Ukrainian soldiers who had been in the United States for training when Russia invaded Ukraine on Feb. 24 received coaching on armed, unmanned Switchblade aircraft, as well as on coastal defense surface drones that Biden just approved Wednesday. Those troops have returned to Ukraine, said a senior defense official, who spoke on the condition of anonymity under rules set by the Pentagon.

It is clear that some of the earlier self-imposed limitations on U.S. military aid to Ukraine have changed, said Elias Yousif, a research analyst at the Stimson Center in Washington. But there are some steps that the Biden administration still deems too risky in terms of provoking conflict with Russia, Yousif said, including facilitating the transfer of fighter jets to Ukraine.

The administration is "walking a tightrope, where it doesn't know the amount of tension it has to work with," Yousif said.

Meanwhile, Finland and Sweden are reconsidering their status as militarily nonaligned nations in the aftermath of Russia's invasion of Ukraine, leading to escalated warnings from Russia.

Dmitry Medvedev, an ally of Putin who serves as deputy chairman of Russia's Security Council, said Thursday that NATO expansion would lead Moscow to strengthen air, land and naval forces to "balance" military capability in the region.

"If Sweden and Finland join NATO, the length of the land borders of the alliance with the Russian Federation will more than double. Naturally, these boundaries will have to be strengthened," he wrote on Telegram.

"There can be no more talk of any nuclear-free status for the Baltic," Medvedev said.

Russia also accused Ukraine's military of striking residential buildings in a Russian village near the border, leaving seven people injured.

Two Ukrainian combat helicopters entered Russian airspace and carried out "at least six strikes" in the village of Klimovo, Russia's Investigative Committee said. The federal agency said six houses were damaged but provided few other details. Ukrainian authorities had no immediate response to the accusations.

Russia has accused Ukraine of several strikes on its territory since the war began. In early April, Moscow claimed Ukraine attacked a fuel depot in Belgorod, another border region. Ukrainian officials would not confirm or deny the attack at the time.

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Rauhala reported from Brussels. The Washington Post's Annabelle Timsit and Adela Suliman in London; Mary Ilyushina in Riga, Latvia; Annabelle Chapman in Warsaw; Amanda Coletta in Toronto; and John Hudson, Christine Armario, Tyler Pager, Kim Bellware and Paulina Firozi in Washington contributed to this report.