The United States has responded to Russian troops’ latest gains by rushing some $US200 million ($A274 million) in armaments to support war-ravaged Ukraine.

The consignment of small arms, anti-tank rounds and surface-to-air missiles is a direct response to Ukrainian officials’ pleas for more equipment to defend against heavy shelling by Russian forces.

President Joe Biden authorised the additional security assistance, the White House said, paving the way for the “immediate” shipment of fresh military equipment to Ukraine, a senior administration official said.

Biden’s decision brings total US security aid provided to Ukraine to $US1.2 billion ($A1.6 billion) since January 2021, and to $US3.2 billion ($A4.4 billion) since 2014, when Russia annexed the Crimea region of Ukraine, according to senior administration officials.

In a memorandum to Secretary of State Antony Blinken, Biden directed that up to $US200 million ($A274 million) allocated through the Foreign Assistance Act be designated for Ukraine’s defence.

‘Iron will and profound courage’

Blinken said he had authorised a fourth drawdown of US defence stocks, in line with Biden’s directive, “to help Ukraine meet the armoured, airborne, and other threats it is facing” as the war entered a third week.

He lauded the “great skill, iron will and profound courage” being shown by the Ukrainian armed forces and citizens, and said the US would also continue to provide humanitarian assistance to those in need.

“The United States and our allies and partners stand in solidarity with the people and government of Ukraine in the face of the Kremlin’s aggression,” he said in a statement.

“The international community is united and determined to hold (Russian President Vladimir) Putin accountable.”

Russia says it is engaged in a “special military operation” that it says is not designed to occupy territory but to destroy its neighbour’s military capabilities and “de-Nazify” Ukraine.

The funds “will provide immediate military assistance to Ukraine, including anti-armour, anti-aircraft systems, and small arms in support of Ukraine’s front-line defenders,” one of the US officials said, speaking on condition of anonymity.

Ukraine has been asking for more Javelin anti-tank weapons and Stinger missiles to shoot down aircraft.

Ukraine’s foreign minister Dmytro Kuleba on Saturday underscored Ukraine’s need for additional military supplies in an interview with the non-profit Renew Democracy Initiative.

The US has drawn from US weapons stocks to supply Ukraine repeatedly, beginning in the autumn of 2021 and then again in December and February.

The last batch of US weapons provided in February included anti-armour, small arms, body armour and various munitions, according to the Pentagon, as well as anti-aircraft systems.

On Thursday night the US Congress approved $US13.6 billion ($A18.6 billion) in emergency aid for Ukraine as part of a $US1.5 trillion ($A2.1 trillion) measure to fund the US government through September.

-with AAP