The Republican-led House of Representatives has been blocking $60 billion in assistance for Ukraine and the United States has warned that a recent $300 million package would last only a few weeks.

The "United States will not let Ukraine fail", Austin said at the opening of a meeting in Germany of dozens of Ukraine's international supporters.

"We remain determined to provide Ukraine with the resources that it needs to resist the Kremlin's aggression," he said.

Washington announced the $300 million package for Ukraine last week, but Austin said it was possible only due to savings on recent purchases.

"We were only able to support this much-needed package by identifying some unanticipated contract savings", he said.

Ukrainian President Volodymyr Zelensky said Monday that it was "critically important for us that the Congress soon completes all the necessary procedures and makes a final decision" on aid for Kyiv.

Austin expressed hope Tuesday that the US Congress would approve the additional assistance "soon", and said that aiding Ukraine benefits Washington as well as Kyiv.

The "United States stands by Ukraine because it's the right thing to do and because America cares when freedom is at risk. But we also stand by Ukraine because it's crucial to our own security", he said at a news conference.

Speaking alongside Austin, the top US military officer, General Charles "CQ" Brown, said that aiding Ukraine "strengthens NATO. It helps to bolster the defence industrial base of the United States, Europe and the world."

"Russia's plan is to wait out Western will to support Ukraine. This coalition must not let that strategy work", he said.

But the delay in providing additional aid is already hampering Kyiv's forces.

Russia said Tuesday that its troops had again made gains in eastern Ukraine, building on the capture of Avdiivka a month ago.

Ukraine's troops meanwhile were "having to pay attention to their supply rates and how they execute," Brown told journalists en route to the Ramstein meeting.

- 'Incremental gains' -

There is an "incremental kind of back and forth between Ukraine and Russia", with "incremental gains on both sides", Brown said.

But he noted that "even as the Russians have gained territory, they do it at a pretty big cost in number of casualties, like in personnel, but also in number of pieces of equipment that are being taken out".

Austin said Tuesday that "Russia has paid a staggering cost for (President Vladimir) Putin's imperial dreams", using "up to $211 billion to equip, deploy, maintain, and sustain its imperial aggression against Ukraine".

"At least 315,000 Russian troops have been killed or wounded" since Russia launched its invasion in 2022, Austin said, adding that Ukraine had also "sunk, destroyed, or damaged some 20 medium-to-large Russian navy vessels".

The sinkings have been an embarrassment for Moscow and Russian state media confirmed Tuesday that the head of its navy had been replaced.

Austin and other US officials have spearheaded the push for international support for Ukraine, quickly forging a coalition to back Kyiv after Russia invaded and coordinating aid from dozens of countries.

Speaking on the sidelines of the Ukraine meeting, German Defence Minister Boris Pistorius said Berlin had offered another half a billion euros' worth of military aid to Ukraine.

"That includes 10,000 rounds of ammunition from the stock of our armed forces," he told journalists.

Germany is Ukraine's second-biggest donor of military support after Washington, which has committed tens of billions of dollars to aid Kyiv since February 2022.

But Chancellor Olaf Scholz has in recent months come under intense fire for his refusal to deliver long-range Taurus missiles to Ukraine.

Speaking at a forum on Tuesday, Scholz hit out at domestic critics, saying the debate was "embarrassing for us as a country".

Germany fears that Taurus missiles could be used to hit targets deep within Russia.

Scholz had repeatedly insisted that "German soldiers must not be linked at any point or place to the targets that this system reaches".