The Pentagon is defending its preparations in response to the threat of a Russian invasion of Ukraine, with a top spokesman on Thursday highlighting that the United States has provided millions of dollars in weapons to Kyiv and providing new details about U.S. military forces that could deploy to Eastern Europe to bolster security there. 

Pentagon spokesman John Kirby said the United States has been monitoring Russian President Vladimir Putin's military buildup along the Ukrainian border for months. More than 100,000 Russian troops are amassed, including some in neighboring Belarus.

"I take issue with the idea that this is sort of 11th-hour, Hail Mary-pass-throwing stuff," Kirby said. "We've been talking about this now for a couple of months, what we've been seeing on the ground."

The comments came as the U.S. military prepared to potentially send thousands of troops from the United States to Europe. Kirby identified for the first time that elements of the 82nd Airborne Division and XVIII Airborne Corps from Fort Bragg, N.C., the 101st Airborne Division from Fort Campbell, Ky., and the 4th Infantry Division from Fort Carson, Colo., were among an initial force of 8,500 troops that were put on high alert this week and could be among the first to go.

Other units also have been put on a heightened alert status, Kirby said. He declined to name them but said they are located at bases that include Fort Hood, Texas; Joint Base Lewis-McChord in Washington state; Fort Polk, La.; and Davis-Monthan Air Force Base in Arizona. Troops from those units could provide medical, aviation and logistics support in addition to combat power, he said.

Underscoring the situation's sensitivities, Kirby declined to say which units from those bases could deploy. But they could significantly enhance NATO's capabilities. Davis-Monthan, for instance, is home to five Air Force squadrons of A-10 tank-killing attack jets. A defense official, speaking on the condition of anonymity because of the issue's sensitivity, said that the Pentagon has become increasingly careful about the information it releases concerning U.S. forces in Europe, as the administration seeks to emphasize that diplomacy is still an option in the crisis.

The Pentagon's deliberations over how to respond to the crisis come as it balances how to show resolve, work with European allies, avoid a potential quagmire and keep focus on security concerns posed elsewhere by China, according to current and former U.S. officials. President Joe Biden has ruled out any U.S. troops fighting in Ukraine, but an array of other options are on the table.

Robert Brown, a retired Army general with experience in Europe, said he has not heard "anybody in their right mind who thinks we would go [into] Ukraine." But there is a desire, he said, to look for ways to strengthen the military alliance in Europe in light of Russian President Vladimir Putin's recent decisions and unpredictability.

"Folks were nervous before this about Russia's aggressive actions," Brown said. "Now once they cross that border into Ukraine, who's to say they won't keep going? I wouldn't put it past Putin in a heartbeat."

Russia's current buildup follows its 2014 seizure of Ukraine's Crimean Peninsula, which Russia annexed through force and continues to control, and a 2008 invasion of the Republic of Georgia. Biden has promised allies that the United States considers its obligations to fellow NATO countries "sacred." Under the terms of their treaty, which was signed by original members in the wake of World War II, an attack on one member must be consider an attack on all of them.

But that doesn't mean that United States needs to shoulder most of the load, Republicans and Democrats agree.

"It's on us to go to the allies and say: You're going to have step up and do more this time, no kidding," said Jim Townsend, a senior Pentagon official during the Obama administration.

NATO has a response force that includes up to 40,000 troops from member nations, including the United States. If all NATO members agree to deploy the force, Townsend said, it would be a "big deal" for the alliance, which could shore up defenses in the Baltic states of Estonia, Latvia and Lithuania. Once a part of the Soviet Union, they joined NATO in the 1990s.

"It would be a real signal that the allies themselves have cranked up the pressure on the military side," Townsend said.

Launching such a NATO deployment, however, would require consent from all members, and there is reason to believe securing that could be difficult. Germany, a longtime U.S. ally that obtains natural gas from Russia, is seen as a potential holdout after Berlin declined to send lethal arms to help the Ukrainian military. If Germany does not consent, the United States could deploy troops independently to countries on Europe's eastern flank that ask for additional security, Townsend said.

Kirby, speaking at the Pentagon, said the United States also could reposition some of the more than 60,000 U.S. troops permanently stationed in Europe. About 200 Florida National Guard members also are deployed in Ukraine to advise its military, but they are west of Kyiv, far from the border with Russia. Kirby said the Pentagon believes they could be withdrawn quickly if required.

At sea, the U.S. Navy has several previously scheduled deployments that could be a factor in the Ukraine crisis, a defense official said. They include surveillance flights from an airfield in Sigonella, Italy, by P-8 jets designed to hunt for submarines below the ocean's surface.

The Navy also has joined this month with allies in a sprawling, previously scheduled NATO exercise in the Mediterranean Sea that includes the aircraft carrier USS Harry S. Truman and its accompanying escort ships, collectively carrying nearly 6,000 U.S. troops and dozens of strike aircraft.

The Navy has other vessels in the Mediterranean and Adriatic seas, as well, said Cmdr. Richlyn Ivey, a military spokeswoman. U.S. warships also regularly sail into the Black Sea, which abuts Russia and Ukraine, but no vessels are there at the moment. In December, U.S. European Command said that the destroyer USS Arleigh Burke had departed the Black Sea and was making a port stop in Turkey.

The Marines, though not among the units Kirby identified Thursday, but could get involved if requested, a senior Marine Corps official said. The service has Marines preparing for several deployments to the region this year, including a force of hundreds due to arrive in Norway for exercises in March, the official said.

"We would respond to tasking if told to do so," the official said. "We could definitely do more, but we have to be asked."

As preparations continue, some have questioned whether the Biden administration is disregarding security concerns posed by China, which the administration has described as its "pacing challenge."

Elbridge Colby, a senior defense official during the Trump administration, said that he believes the Biden administration should clearly state that it supports NATO and will continue to be a part of it, but warn its allies that the United States must "truly prioritize" getting ready for China.

"People say, 'Well, of course they need to focus on Europe now,'" Colby said. "Well, no, they don't. Yes, there are going to be crises. But the fact that there are crises doesn't change in the slightest the underlying facts, which is that Asia is the most important region, and China our top threat."

Kirby said that the United States will continue to "walk and chew gum at the same time."

"There's a lot on our plate, and we're focused on all of it," he said. "Just because right now, one issue obviously is certainly capturing the attention of the world community doesn't mean that we're not equally pursuing and focused on other threats and challenges."