Washington DC - The White House announced Tuesday a stopgap $300 million package of vital military aid to Ukraine, sidestepping Republicans in Congress amid fears that Russia's invasion is gaining ground.

The shipment, the first in months from Kyiv's biggest ally, will include anti-aircraft missiles, ammunition, artillery rounds, and some armored vehicles, officials said.

The announcement comes as President Joe Biden hosts Poland's President Andrzej Duda and Prime Minister Donald Tusk at the White House to reassure them of US support for NATO and Ukraine.

"Today, on behalf of President Biden, I'm announcing an emergency package of security assistance and $300 million worth of weapons and equipment to address some of Ukraine's pressing needs," National Security Advisor Jake Sullivan told reporters.

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"The world is watching. The clock is ticking, and we need to see action as rapidly as possible."

The announcement comes as Republican lawmakers have been blocking Biden's request for a $60 billion aid package for Ukraine, with the Democratic president's rival, Donald Trump, urging them not to give him a legislative win in an election year.

US defense officials said they had managed to find the $300 million for Ukraine by making savings on recent purchases by the Pentagon but stressed the need for Congress to pass more aid.

"It is a relatively small package to give Ukraine the minimum of what it needs for a short amount of time," a senior defense official told reporters, speaking on condition of anonymity.

“We had savings come in that will allow us to offset the cost of a new drawdown package," the official added. "This is a bit of a unique occurrence... so we do need the House to act."

Alarm is growing among US allies as Congress blocks aid for Ukraine and after Trump threatened to cut funding for Kyiv if elected while encouraging Russia to invade NATO countries that fail to meet defense spending goals.

Moscow is meanwhile pushing forward with an offensive in the east of Ukraine, where it has made a series of recent gains as the war grinds on into its third year.

The head of the Central Intelligence Agency said on Tuesday that Ukraine was "running out of time."

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"Ukraine is not running out of courage and tenacity – they're running out of ammunition. And we're running out of time to help them," CIA Director William Burns told Congress.

A US intelligence assessment published Monday said that the "deadlock plays to Russia's strategic military advantages and is increasingly shifting the momentum in Moscow's favor."

Biden has previously warned that Russian President Vladimir Putin "won't stop" with Ukraine.

The war has sparked fears in Poland and other eastern European countries that an aggressive Russia could strike a NATO nation next if it is allowed to win in Ukraine. Before leaving for Washington, Poland's Duda said that NATO members should increase their defense spending to 3% of GDP in response to Russia's war in Ukraine.

The Western defense alliance currently has a defense spending target of 2% of gross domestic product, though Poland already spends around 4% and the United States spends 3.5%.