The United States has announced a $2.5 billion security assistance package for Ukraine as Washington races to provide aid to Kiev before President-elect Donald Trump takes office.

Trump's November election victory has cast doubt on the future of American aid for Ukraine, providing a limited window for billions of dollars in already authorised assistance to be disbursed before he is sworn in next month.

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

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

"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," President Joe Biden said in a statement.

The drawdowns from the defence department shelves will involve 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 a separate statement from the US State Department.

"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.

The latest 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 who has repeatedly criticised US assistance for Kiev and claimed he could secure a ceasefire within hours takes over in January.