WASHINGTON — The White House on Thursday denounced the poisoning of Russian opposition leader Alexei Navalny and suggested that the United States might retaliate if the Kremlin is to blame, but President Donald Trump has failed to repudiate the attack himself, prompting criticism that he is once again being soft on Russian President Vladimir Putin.

White House press secretary Kayleigh McEnany called the poisoning "completely reprehensible" but did not address a question about whether Trump has "made his voice known to the Russian government."

It was the strongest U.S. condemnation yet of the attack two weeks ago using what a German military lab says was a banned chemical weapon. Navalny survived and is now under treatment in Germany.

Still, Trump himself has remained almost entirely silent on the attack on the most prominent domestic critic of Putin. Trump had said nothing on the matter since last Thursday, when he told reporters the United States was looking into the then-unconfirmed reports that Navalny had been poisoned.

The silence led Democratic presidential nominee Joe Biden to say Trump is complicit in a Kremlin attempt to silence a political opponent, and others of Trump's critics to say he is beholden to Putin, an authoritarian leader Trump has said he admires.

Trump has a pattern of publicly declining to confront Putin or to directly accuse the Russian leader of wrongdoing.

The president has said he did not question Putin about intelligence reports that Russia has offered to pay bounties to militants to kill American forces in Afghanistan, and he initially refused to pin blame on Russia for an earlier poisoning attempt, in Britain.

He publicly took Putin's word over that of U.S. intelligence agencies that had concluded Russia interfered in the 2016 presidential election to help Trump. Intelligence agencies have found that Russia is trying to do the same thing ahead of Trump's effort to win reelection this year.

"We're deeply troubled by the results" of a German military analysis of the agent used to poison Navalny, McEnany said Thursday, adding that Russia "has used chemical nerve agents in the past."

Germany had announced that tests on Navalny showed he had been poisoned by the chemical nerve agent related to Novichok, which was developed under the Soviet Union. Navalny's wife and supporters say he was poisoned at an airport cafe while traveling in Siberia, where he has worked to support opposition political candidates.

Navalny remains in a medically induced coma.

Biden issued a statement Wednesday blaming Moscow and saying that Trump's "silence is complicity."

"The Kremlin no doubt thinks that it can act with impunity. Donald Trump has refused to confront Putin, calling him a 'terrific person,'" the former vice president said.

He pledged to "do what Donald Trump refuses to do: work with our allies and partners to hold the Putin regime accountable for its crimes."

McEnany said the United States would work alongside other countries "to hold those in Russia accountable, wherever the evidence leads, and restrict funds for their malign activities."

That hinted at further financial sanctions, though U.S. officials did not elaborate. The Trump administration has imposed severe financial sanctions on Russia over its 2016 election interference and its invasion of Ukraine, and applied further sanctions in response to the 2018 nerve agent poisoning of former Russian spy Sergei Skripal and his adult daughter Yulia at their home in Britain. Russia denies any involvement.

Trump dragged his feet in condemning the attack on an ally's soil and in assigning blame. Republicans in Congress were among those urging a more direct and forceful response. In the end, Trump appeared to have had little choice but to apply sanctions. Under a 1991 law, he was required to act once the administration determined Russian responsibility for a chemical or biological weapons attack.

Trump faced criticism for his tepid response to the Skripal poisoning. The Washington Post previously reported that in a summer 2018 call with then-British Prime Minister Theresa May, Trump disputed her intelligence community's conclusion that Putin's government had orchestrated the attempted murder.

"Trump was totally bought into the idea there was credible doubt about the poisoning," one person briefed on the call told The Post, speaking on the condition of anonymity to describe the private conversation. "A solid 10 minutes of the conversation is spent with May saying it's highly likely and him saying he's not sure."

The White House did not immediately comment when the German findings were released Wednesday, and Trump remains nearly alone among major Western leaders in not addressing the issue personally.

British Prime Minister Boris Johnson, a Trump confidant, had tweeted Wednesday that the "outrageous" attack demands answers from Russia.

"We have seen firsthand the deadly consequences of Novichok in the UK," Johnson tweeted. "The Russian government must now explain what happened to Mr Navalny - we will work with international partners to ensure justice is done."

German Chancellor Angela Merkel called the poisoning an attempted murder and also demanded an explanation from Moscow. French President Emmanuel Macron joined Merkel in offering asylum and medical help.

"We are supposed to be the leader of free democracies, the leader of the free world," said Evelyn Farkas, a former senior Pentagon official focused on Russia in the Obama administration.

"It's unacceptable that he's not out there speaking for the human rights of this man, who was faced with a state assault, an attempted assassination," she said. "For some reason, our president thinks its more important to not upset Vladimir Putin and to avoid saying anything critical about him than to stand up for the lives of Americans and others."

Russia denies harming Navalny. Maria Zakharova, a spokeswoman for the Russian Foreign Ministry, said the German announcement was an "information campaign" against Russia.

Navalny's spokeswoman, Kira Yarmysh, responded on Twitter by saying: "What hard work for a person. To appear before people with a stone face and talk rubbish."

Navalny has been the foremost opposition activist in Russia for nearly a decade, since leading protests in late 2011 against flawed parliamentary elections that marked the biggest homegrown opposition threat to Putin's rule to date. He coined the populist rallying cry "crooks and thieves" as an opposition chant to describe the inner circle of people around Putin profiting from their proximity to power with offshore accounts and lavish homes at home and abroad.

Others in the Trump administration have not been as quiet as Trump himself.

Trump's envoy to the Organization for Security and Cooperation in Europe, James Gilmore III, confronted his Russian counterpart at a meeting in Vienna on Thursday.

"We urge Russia to cooperate fully with the international community's investigation into the poisoning. Those responsible, both those who committed this crime and those who ordered it, must be held accountable," Gilmore said.

National security adviser Robert O'Brien said last week that Navalny's illness was of deep concern to the United States and could affect U.S.-Russian relations.

"He's a very courageous man. He is a very courageous politician to have stood up to Putin inside Russia, and our thoughts and our prayers are with him and his family," O'Brien said in an interview on Fox News.

Sen. Robert Menendez of New Jersey, the top Democrat on the Senate Foreign Relations Committee, called on the Trump administration to determine whether Russia was responsible for the poisoning, and if so, invoke the same 1991 law used in the Skripal case — the Chemical and Biological Weapons Control and Warfare Elimination Act — to impose sanctions.

"The president of the United States must coordinate a strong international response to this latest aggression along with our partners and allies. Silence will be interpreted as a tacit approval for these types of attacks to continue," Menendez said in a statement.

On Thursday, the Organization for the Prohibition of Chemical Weapons said that any poisoning of an individual through a nerve agent is considered a use of chemical weapons.

"Such an allegation is a matter of grave concern," the organization said. "States parties to the Chemical Weapons Convention deem the use of chemical weapons by anyone under any circumstances as reprehensible and wholly contrary to the legal norms established by the international community."

Andrew Weber, a senior fellow at the Council on Strategic Risks and a former top Pentagon official overseeing chemical and biological weapons defense during the Obama administration, said the limited use of chemical weapons in assassinations and tactical warfare was weakening international norms and the United States must take a leading role in defending them.

"Russia's repeated use of chemical weapons makes it urgent to strengthen the Chemical Weapons Convention," Weber said. He said the United States should send a strong signal of support for Merkel's announcement and stand against the creeping usage of these weapons.

"That is extraordinarily dangerous and we need to fight back against that," Weber said.