The imminent return of Trump, who has repeatedly criticized aid for Ukraine, has cast doubt on American support continuing at similar levels as under outgoing President Joe Biden.

With Trump's swearing in just three weeks away, only a limited time window remained for billions of dollars in already authorized assistance to be disbursed.

"Today, I am proud to announce nearly $2.5 billion in security assistance for Ukraine, as the Ukrainian people continue to defend their independence and freedom from Russian aggression," Biden said in a statement.

The package includes a $1.25 billion military "drawdown," which allows the Pentagon to take weapons from US stocks and send them quickly to the battlefield.

An additional $1.22 billion will be funded via the Ukraine Security Assistance Initiative, under which military equipment is procured from the defense industry or partners.

The latest drawdown includes drones, ammunition for High Mobility Artillery Rocket Systems (HIMARS), optically guided missiles, anti-tank weapons systems, air-to-ground munitions and spare parts, according to the US State Department.

The Treasury Department meanwhile announced the disbursement of $3.4 billion in direct budgetary support for Ukraine.

Biden said his administration has now allocated all the Ukraine aid money appropriated by Congress in the spring of this year after arduous negotiations.

The president said he has ordered his administration to keep sending Ukraine as much military aid as possible, including older US equipment, and rushing it to the battlefield as Ukraine loses territory to the Russian troops that invaded in February 2022.

"Together with the security assistance America is providing Ukraine and Treasury's actions to further tighten sanctions on Russia's war machine, we will continue to do everything in our power to position Ukraine to achieve a just peace," Treasury Secretary Janet Yellen said in a statement.

- 'Peace in 2025'? -

As Ukraine heads into winter, the Pentagon is delivering artillery rounds, rockets and armored vehicles, Biden said.

"The United States and more than 50 nations stand united to ensure Ukraine has the capabilities it needs to defend itself against Russia's aggression," Secretary of State Antony Blinken said.

Ukrainian President Volodymyr Zelensky said the aid came at a critical moment as Russia intensifies its assaults, "even resorting to involving North Korean soldiers and continuing to receive weapons from North Korea and Iran."

The Ukrainian leader also called on Kyiv's Western backers to continue boosting his country's defenses in a bid to secure an end to the conflict next year.

"We must continue moving toward peace through strength to achieve our common goal of peace in 2025 -- a goal shared by Ukraine and all its partners," Zelensky said in a social media post.

The latest US assistance for Ukraine follows an announcement at the beginning of the month of a nearly $1 billion tranche of drones, ammunition and equipment.

The outgoing Biden administration is working to get as much aid as possible to Ukraine before Trump takes office on January 20.

Trump has often spoken admiringly of Russian President Vladimir Putin and asserted that he himself can end the war in a day, without saying how.

This has prompted fears that the United States will impose war-ending terms dictated by Russia.