WASHINGTON, March 12 (Reuters) - The United States will send a new military aid package for Ukraine worth $300 million, President Joe Biden's administration said on Tuesday, the first such move in months as additional funds for Kyiv remain blocked by Republican leaders in Congress.

The White House has been scrambling to find ways to send more military assistance given the situation on the battlefield and the resistance to the funding from Republican hardliners.

U.S. National Security Advisor Jake Sullivan said the funding was coming from unanticipated cost savings from Pentagon contracts and would be used for artillery rounds and munitions for High Mobility Artillery Rocket Systems (HIMARS).

"This ammunition will keep Ukraine's guns firing for a period, but only a short period," Sullivan told reporters, adding it may only be helpful to Ukraine for a couple of weeks.

"It is nowhere near enough to meet Ukraine's battlefield needs and it will not prevent Ukraine from running out of ammunition," Sullivan said.

The new weapons package was first reported by Reuters earlier on Tuesday.

Pentagon Press Secretary Major General Pat Ryder said the package included anti-aircraft missiles and artillery rounds. He said providing weapons to Ukraine through Pentagon contract savings was likely a one-time situation and not a sustainable way of funding Kyiv.

The last drawdown was in December 2023 when funds to replenish stocks fell to zero.

U.S. officials have also looked at options for seizing some $285 billion in Russian assets immobilized in 2022 and using the money to pay for Ukraine weaponry.

The announcement came as Poland’s president and prime minister meet President Joe Biden at the White House later on Tuesday to talk about ways to bolster support for Ukraine.

"Financial support for Ukraine is cheap considering what the other support could be," Polish President Andrzej Duda told reporters.

Using the funds that have been returned to replenish stocks opens a narrow window to allow more aid to be sent from existing stocks as the Biden administration waits for supplemental funding to be passed by lawmakers.

Biden, a Democrat, has backed military aid to Ukraine since Russia's full-scale invasion in 2022, while his likely Republican opponent in the Nov. 5 U.S. election, former President Donald Trump, has a more isolationist stance.

Republican House of Representatives Speaker Mike Johnson, an ally of Trump, has so far refused to call a vote on a bill that would provide $60 billion more for Ukraine.

The measure has passed the Democratic-run Senate, and both Republicans and Democrats in the House say it would pass if the chamber's Republican leaders allowed a vote.

Leaders of U.S. intelligence agencies pressed members of the House of Representatives on Tuesday to approve additional military assistance for Ukraine, saying it would not only boost Kyiv as it fights Russia but discourage Chinese aggression.

Ukraine President Volodymyr Zelenskiy said on Monday that the situation along the front of the country's war with Russia was the best in three months, with Moscow's troops no longer advancing after their capture last month of the eastern city of Avdiivka.

Zelenskiy, in an interview with France's BFM television, said Ukraine had improved its strategic position despite shortages of weaponry, but suggested the situation could change again if new supplies were not forthcoming.

He said earlier that Russia is preparing a new offensive against Ukraine starting in late May or summer. Zelenskiy has said 31,000 Ukrainian soldiers have been killed since February 2022.

Russia's capture of Avdiivka gave the Kremlin's forces breathing room in defending the Russian-held regional center of Donetsk, 20 km (12 miles) to the east.

Earlier this month, a top military commander said Ukrainian troops were forced to leave several settlements neighboring Avdiivka due to Russia's continued offensive amid its own depleting stockpiles of munitions.

Denmark will provide a new military aid package including Caesar artillery systems and ammunition to Ukraine worth around 2.3 billion Danish crowns ($336.6 million), the Danish Defence Ministry said on Tuesday.

European Union countries are set to agree on a new 5 billion-euro ($5.46 billion) top-up to a fund used to finance military shipments to Ukraine, the Financial Times reported on Tuesday, citing four officials briefed on the discussions.