"We can't afford to let up and we can't lose steam. The stakes are too high. Ukraine is facing a pivotal moment on the battlefield," Austin said at a meeting in Brussels with some 50 countries backing Ukraine.

"We must intensify our shared commitment to Ukraine's self defence, and we must push ourselves even harder to ensure that Ukraine can defend itself, its citizens and its territory."

Austin said that Moscow's attack on its pro-Western neighbour "isn't just a danger to Ukraine -- it's a menace to European security".

"So we must continue to rise to meet this challenge," he said, sitting next to Ukraine's defence minister Oleksiy Reznikov.

The West has poured major supplies of arms into Ukraine to help it fight the Kremlin forces but Ukraine complains it has only received a fraction of what it needs and is clamouring for heavier weaponry.

Austin said Washington and Kyiv were "working in lockstep to meet Ukraine's requests for new capabilities, especially its need for long-range fires, armour and coastal defence."

He said deliveries of longer-range Himars rocket artillery and similar systems from Britain would "significantly boost Ukraine's capability".

In addition NATO allies, including the US, were providing training to Kyiv's forces to handle the modern weaponry.

"So we've got a lot done thus far, but we don't have any time to waste," he said.

"We're here to dig in our spurs. We're going to deepen our support for the Ukrainian armed forces in today's fight."