U.S. Defense Secretary Lloyd Austin on Tuesday warned that Ukraine's survival was in danger and sought to convince allies that the United States was committed to Kyiv, even as Washington

has essentially run out of money to continue arming Ukrainian forces.

Republican House of Representatives Speaker Mike Johnson has so far refused to call a vote on a bill that would provide $60 billion more for Ukraine and the White House has been scrambling to find ways to send assistance to Kyiv, which has been battling Russian forces for more than two years.

Austin is leading the monthly meeting known as the Ukraine defense contact group (UDCG), held at Ramstein Air Base in Germany, of about 50 allies that have been militarily supporting Ukraine.

"Ukraine's survival is in danger," Austin said in a press conference after the meeting.

"Our allies and partners continue to step up, the United States must also," he added.

Austin, who is traveling for the first time this year since prostate cancer treatment, did not say how Washington would support Ukraine without additional funding.

Officials say the lack of funding available is already having an impact on the ground in Ukraine and Ukrainian forces are having to manage scarce resources.

"I think our allies are acutely aware of our funding situation and the Ukrainians more so than anyone because of the shortages that are resulting from us not being able to supply them," a senior U.S. defense official, speaking on the condition of anonymity, said.

Ukraine's President Volodymyr Zelenskiy appealed to Kyiv's allies on Tuesday to supply more air defenses, saying Russia had launched 130 missiles, more than 320 attack drones and almost 900 guided bombs in attacks this month alone.

Last week the Biden administration said it would send $300 million in military assistance to Ukraine, but added that it was an extraordinary move after unexpected savings from military contracts the Pentagon had made.

Officials have not ruled out that they could find additional savings, but they say that amount would not be enough to make up for the lack of Congressional action.

Experts say that Austin will face a skeptical audience in Europe.

"It's becoming harder and harder for U.S. leaders to travel to Europe, with the message that the United States is committed to Ukraine in the long-term," Rachel Rizzo, a senior fellow at the Atlantic Council's Europe Center in Washington, said.

"The message of this long-term financial, military, economic commitment flies in the face of the reality of what's happening on Capitol Hill," Rizzo added.

At a joint press conference in Berlin on Friday, German Chancellor Olaf Scholz, French President Emmanuel Macron and Polish Prime Minister Donald Tusk reaffirmed their support for Ukraine, whose ammunition-starved troops face their toughest battles since the early days of Russia's invasion two years ago.

German Defence Minister Boris Pistorius announced on Tuesday a 500 million euro ($542.55 million) aid package for Ukraine which includes 10,000 rounds of ammunition and said the United States was still a reliable partner.

"There are particularities in the political systems, and we have to deal with that," Pistorius added.

European support has become increasingly key with Biden unable to get a big Ukraine aid package through Congress, and much of his foreign policy energy is focused on the war in Gaza.

But U.S. officials say that the reality is that without the United States, European support for Ukraine will not be enough to fend of Russian forces.

"There isn't a way that our allies can really combine forces to make up for the lack of U.S. support," the senior U.S. defense official said.

Ukraine President Volodymyr Zelenskiy has said that Russia is preparing a new offensive against Ukraine starting in late May or summer.