An aerial photograph taken with a drone shows workers digging graves for the victims of an air strike in the Groza village of Ukraine's Kharkiv region. AFP

US President Joe Biden announced on Tuesday a $300 million emergency weapons package to prop up Ukraine while Congress blocks further aid, as Poland's leaders visited the White House to warn of the growing threat from Russia.

Biden said the stopgap shipment of missiles, shells and ammunition for Kyiv was "not nearly enough" and would run out in a couple of weeks, leaving Ukraine outgunned by Russian President Vladimir Putin's invading forces.

The Democrat urged Republicans to stop blocking his larger, $60 billion aid package for Ukraine, which has been caught up in a bitter partisan fight ahead of a likely election rematch against Donald Trump in November.

"We must act before it literally is too late," said Biden, 81, as he met Poland's President Andrzej Duda and Prime Minister Donald Tusk at the White House.

"Russia won't stop at Ukraine. Putin will keep going, putting Europe, the United States and the entire free world at risk," added Biden, backed by Polish and US flags and flanked by his top military and diplomatic officials.

The White House said the $300 million package, the first since December, was made possible by using money that the Pentagon has saved on other purchases, thus allowing Biden to bypass the Republican-controlled House of Representatives.

But White House National Security Advisor Jake Sullivan said Ukraine's battle was now in one of its most perilous phases since Russia's February 2022 invasion.

Moscow has made a series of recent gains in eastern Ukraine after months of stalemate, sparking growing Western fears that it could be nearing a breakthrough as the war enters its third year.

Sullivan said the $300 million emergency package was "nowhere near enough to meet Ukraine's battlefield needs and it will not prevent Ukraine from running out of ammunition in the weeks to come."

The shipment would include long-range US-made HIMARS rockets, anti-aircraft and anti-tank weapons, artillery shells and small arms ammunition, the Pentagon said.

The head of the Central Intelligence Agency warned separately of the price of inaction.

"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.

Poland, which borders Ukraine and hosted around one million refugees from the war, is among the NATO allies that have been watching the paralysis in Washington with alarm.

Allies have also been concerned by recent threats by Trump to cut funding for Kyiv if elected in November. He has also encouraged Russia to invade NATO countries that fail to meet defense spending goals.

The Polish leaders were visiting on the 25th anniversary of the day that Poland, the Czech Republic and Hungary joined NATO, less than a decade after emerging from Soviet control.

"The only message we should send to Moscow is that the West is united more than ever before when it comes to Ukraine," Polish Premier Tusk told reporters.

During the Polish leaders' visit, the US State Department said it was approving nearly $3.5 billion in missile sales to Poland.

President Duda said that NATO members should increase their defense spending to three percent of GDP from the current target of two percent in response to Russia's war in Ukraine.

Poland spends the most of any nation in the Western defense alliance -- around four percent -- while the United States spends 3.5 percent.

"Russian imperialism today must not be allowed to disrupt this stability and peaceful existence of Europe," Duda told reporters.

Pro-EU Premier Tusk meanwhile tried to ease fears that his running feud with the right-wing Polish president would affect Warsaw's commitment to Ukraine.

"Poland will be a solid and lasting member of the transatlantic community no matter who wins the elections in our country," Tusk said.