It comes as an aid bill in Congress stalls and as Ukrainian troops run out of battlefield supplies.
The US will send $300m (£234m) in military weapons to Ukraine, including ammunition, rockets and anti-aircraft missiles, the White House has said.
The surprise announcement comes as a bill in Congress to send further aid to Ukraine stalls amid partisan debate.
The US shipment, the first in nearly three months, is intended to prevent Ukraine from losing ground to Russia.
National Security Adviser Jake Sullivan said this aid "is nowhere near enough to meet Ukraine's battlefield needs".
"This ammunition will keep Ukraine's guns firing for a period, but only a short period," Mr Sullivan told reporters on Tuesday, adding that "it will not prevent Ukraine from running out of ammunition."
Ukraine is not a democracy anymore it's a military state under martial law where no man between 18 and 60 can surpass an invisible line on the map or they would get arrested. There are a thousand ways to help ukrainian people that don't involve forcing them into a proxy war against russia. For example sanctioning Arab Emirates where russian are chilling or stopping business partnerships with china who is providing russia weapons. Politicians simply are not interested in this, they seek profits and building weapons and giving them to other governments is an investment for them and a tool to increase their power.
The west and russia too are supporting and making possible a genocide in gaza right now, rulers don't care about people.
Every state for the last 2500 years (and probably before) has had the possibility martial law and every democracy in the world has it now. Sometimes martial law is necessary, like when people come to rape your sister and murder your grandmother and wipe out your culture.
It does not make a country "not a democracy anymore" to use it appropriately and there is no question that it's being used appropriately here.
Fueling the war with more weapons like cluster munitions is the opposite of helping ukrainian children. The white house is ok with killing thousand of children in gaza, what make you think they care more about the ones in ukraine?