The US will send $300m (£234m) in military weapons to Ukraine, including ammunition, rockets and anti-aircraft missiles, the White House has said.
The surprise announcement comes as a bill in Congress to send further aid to Ukraine stalls amid partisan debate.
The US shipment, the first in nearly three months, is intended to prevent Ukraine from losing ground to Russia.
National Security Adviser Jake Sullivan said this aid “is nowhere near enough to meet Ukraine’s battlefield needs”.
“This ammunition will keep Ukraine’s guns firing for a period, but only a short period,” Mr Sullivan told reporters on Tuesday, adding that “it will not prevent Ukraine from running out of ammunition.”
The White House has been appealing to Congress for months to pass a budget that sends aid to Ukraine, as well as Israel and Taiwan.
A $60bn aid bill has already passed the Senate, but has yet to face a vote in the House of Representatives.
Speaker of the House Mike Johnson has so far refused to consider the Senate bill. Mr Johnson, an ally of Donald Trump, has said the House will vote on its own aid bill, but only after Congress passes a budget that overhauls the US immigration system.
On Tuesday, a group of bipartisan lawmakers in the House launched a longshot petition – an attempt to force the House to vote on the Senate bill – using a rare procedural tactic that hasn’t been successfully employed since 2015.
Tuesday’s emergency package of weapons and equipment comes from cost savings made in earlier Ukraine weapon contracts.
The aid announcement came as President Joe Biden hosted Poland’s president and prime minister at the White House in a show of support for Ukraine.
Following the meeting, Polish Prime Minister Donald Tusk told reporters that he hopes Mr Johnson “is already aware that, on his individual decision, depends the fate of millions of people”.
“This is not some political skirmish that only matters here in America,” he said.
“The absence of this positive decision of Mr Johnson will really cost thousands of lives [in Ukraine]. Children. Women. He must be aware of his personal responsibility.”
Also on Tuesday, Denmark announced that it would ship around $336m in ammo and artillery to Ukraine.
Ukraine has lost ground in recent months due an “artificial shortage” of weapons, the country’s president, Volodymr Zelensky, said last month.
