Ukraine’s President Volodymyr Zelensky has made an urgent appeal for more weapons to avoid a “catastrophic” situation in Europe.
An “artificial deficit of weapons” will only help Russia, Mr Zelensky told an international conference in Germany.
Ukrainian troops have been running out of ammunition as vital US support has been held up by supporters of former President Donald Trump in Congress.
Mr Zelensky said he was prepared to tour the front lines with Mr Trump.
“If Mr Trump will come, I am ready to go with him to the front line. What does it mean, the real war, not Instagram, the real war,” the Ukrainian president said.
President Joe Biden assured Mr Zelensky during a phone call on Saturday that the US was committed to supporting Ukraine’s fight against “Russia’s brutal invasion”, the White House said in a statement.
Mr Biden also said that earlier in the day “Ukraine’s military was forced to withdraw from Avdiivka after Ukrainian soldiers had to ration ammunition due to dwindling supplies as a result of congressional inaction, resulting in Russia’s first notable gains in months”.
Avdiivka – a gateway to the Russian-seized Donetsk regional capital in the east – had for months witnessed some of the fiercest fighting before the Ukrainian pull-out announced by the country’s top military commander.
For his part, President Zelensky said in a post on X, formerly Twitter, that the two leaders discussed the current situation on the front line.
He said he was “grateful to have President Biden’s full support”, adding that he hoped Congress would make a “wise decision” on approving a stalled aid package for Ukraine.
US Vice-President Kamala Harris had earlier repeated assurances of support to Kyiv.
The US “can’t play political games” over the military aid, she said at a joint news conference with Mr Zelensky at the Munich Security Conference.
The gathering of world leaders and senior defence officials comes one week before the two-year anniversary of Russia’s full-scale invasion of Ukraine.
The president told delegates Ukrainian efforts were “limited only by the sufficiency and length of range of our strength”.