President Biden is asking Congress for $33bn (£27bn) in military, economic and humanitarian assistance to support Ukraine – although he insisted that the US was not “attacking Russia”.
Mr Biden said it was “critical” for US lawmakers to approve the deal, which he said would help Ukraine defend itself.
The proposal includes more than $20bn in military aid, $8.5bn in economic aid and $3bn in humanitarian aid.
“It’s not cheap,” Mr Biden said on Thursday.
“But caving to aggression is going to be more costly if we allow it to happen.”
Although the US has already announced help for Ukraine, the proposals are a significant ramping up of aid.
President Biden said US military support to Ukraine has so far amounted to 10 anti-tank weapons for every tank that Russia has deployed to Ukraine.
But despite his strong rhetoric, he said the US was not attacking Russia. “We are helping Ukraine defend itself against Russian aggression,” he insisted.
On Thursday, a spokesperson for Russia’s Foreign Ministry said Western military support for Ukraine threatens “the security of the continent”.