The US will send $3.75 billion ($5.9 billion NZD) in military weapons and other aid to Ukraine and its neighbours on NATO's eastern flank, the White House announced Friday, as Russia's invasion of Ukraine grinds on.

The latest tranche of assistance will include for the first time Bradley armoured vehicles for Ukraine. The armoured carrier is used to transport troops to combat and is known as a "tank-killer" because of the anti-tank missile it can fire.

The biggest US assistance package to date for Kyiv includes a $2.85 billion ($4.56 billion NZD) drawdown from the Pentagon's stocks that will be sent directly to Ukraine and $225 million ($356 million NZD) in foreign military financing to build the long-term capacity and support modernisation of Ukraine’s military, according to the White House.

It also includes $682 million ($1.7 billion NZD) in foreign military financing for European allies to help backfill donations of military equipment they've made to Ukraine.

"The war is at a critical point and we must do everything we can to help the Ukrainians resist Russian aggression," White House press secretary Karine Jean-Pierre said in announcing the aid.

The direct assistance for Ukraine includes 50 Bradleys as well as 500 anti-tank missiles and 250,000 rounds of ammunition for the carriers. The US is also sending 100 M113 armoured personnel carriers, 55 mine-resistant ambush protected vehicles, or MRAPS, and 138 Humvees, as well as ammunition for High Mobility Artillery Rocket Systems and air defence systems and other weapons and thousands of rounds of artillery, according to the Pentagon.

White House National Security Council spokesman John Kirby said the Bradleys will be particularly useful to Ukraine in ongoing heavy fighting in largely rural areas of eastern Ukraine.

"It's very much tied to the war that we're seeing on the ground right now and what we anticipate we’ll see throughout the winter months," Kirby said.

Critics have complained that the US has been too slow to provide key weapons such as the Bradleys and battle tanks like the Abrams, saying they could have helped in the fight last year.

At the Pentagon, Laura Cooper, the deputy assistant secretary for Russia and Ukraine, said this is the right time to provide the Bradley. "The Ukrainians have demonstrated a lot of growing proficiency in maintenance and sustainment," she said.

She added that the US-led training set to begin later this month will enable troops to operate, maintain and repair the weapons and that providing tanks, such as the Pentagon's more complex, gas guzzling, heavily armoured M1 Abrams tank, would require more maintenance and other training.