"At my direction, the United States will continue to work relentlessly to strengthen Ukraine’s position in this war over the remainder of my time in office," Biden said in a statement regarding the $US6 billion ($A9.7 billion) pledge.

Biden's announcement includes $US1.25 billion ($A2.01 billion) in military aid drawn from US stockpiles and a $US1.22 billion ($A1.96 billion) Ukraine Security Assistance Initiative (USAI) package, the final USAI package of Biden's time in office.

Under USAI, military equipment is procured from the defence industry or partners, rather than drawn from American stocks, meaning it can take months or years to arrive on the battlefield.

Yellen said in a statement the direct budget assistance, provided in coordination with the US Agency for International Development and the State Department, marked the final disbursement under the 2024 Ukraine Security Supplemental Appropriations Act.

The US Congress has approved $US175 billion ($A282 billion) in total assistance for Ukraine since Russia's invasion of Ukraine nearly three years ago, according to the nonpartisan Committee for a Responsible Federal Budget. Recently the Russians have been using North Korean troops to bolster their fighting position.

North Korean forces are experiencing mass casualties on the front lines of Russia's war against Ukraine, with 1000 of their troops killed or wounded in the last week alone in Russia's Kursk region, White House spokesperson John Kirby said on Friday.

Biden said the new assistance will provide Ukraine with "an immediate influx of capabilities that it continues to use to great effect on the battlefield and longer-term supplies of air defence, artillery, and other critical weapons systems."

Nearly three years into the war, Washington has committed billions of dollars in aid for Ukraine, but it is uncertain if the aid will continue at that pace under Trump, who succeeds Biden on January 20.

Trump has said he wants to bring the war to a swift end.

During the presidential campaign, Trump questioned the level of US involvement in the conflict, suggesting European allies should bear more of the financial burden.

Some of his fellow Republicans - who will control both the House of Representatives and Senate starting next month - have also cooled on sending more aid to Kyiv.

A US official said the $US3.4 billion ($A5.5 billion) in budget funding brings the total in US budget aid to Ukraine to just over $US30 billion ($A48 billion) since Russia's invasion in February 2022. Most of those funds are used to keep Ukraine's government running by paying salaries to teachers and other state employees.

Washington has separately provided approximately $US61.4 billion ($A98.8 billion) in security assistance to Kyiv since the start of the war, according to the Pentagon.

Biden said the Defense Department is in the process of delivering hundreds of thousands of artillery rounds, thousands of rockets, and hundreds of armoured vehicles "which will strengthen Ukraine’s hand as it heads into the winter."

Yellen said continued economic aid for Ukraine was crucial to allow it to maintain government services and continue to defend its sovereignty, warning against moves to cut funding.

"Ukraine’s success is in America’s core national interest," she said, vowing to continue to pressure Moscow with sanctions and to help position Ukraine to achieve a just peace.

"We must not retreat in this effort."

Also on Monday, Ukraine and Russia carried out a new exchange of prisoners of war, with Kyiv bringing home 189 former captives, President Volodymyr Zelenskiy said.

Zelenskiy thanked the United Arab Emirates and other partners for facilitating the swap.

"The return of our people from Russian captivity is always very good news for each of us. And today is one of such days: our team managed to bring 189 Ukrainians home," Zelenskiy said on the Telegram messaging app.

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