Austin is to convene a meeting of dozens of Ukraine's supporters at the Ramstein Air Base in Germany on Thursday, the last such talks before President-elect Donald Trump -- who has been sceptical of assistance for Kyiv -- takes office on January 20.

Ukrainian President Volodymyr Zelensky said on Tuesday that he would attend the gathering.

"US leadership is critical, and we lead by example," Austin told AFP at Ramstein.

"We've not just asked countries to provide security assistance -- we have led the way in every case in terms of the amount of security assistance that we've provided, the speed at which we got that security assistance there," he said.

"It's real important for the entire coalition to continue to provide military aid. Now, the US has led this effort, and hopefully... will continue to do so, because it's not over," Austin said.

The outgoing US administration has been working to get as much aid as possible to Ukraine before Trump -- who has repeatedly criticised US assistance for Kyiv and claimed he could secure a ceasefire within hours -- takes over the presidency.

Trump's comments have triggered fears in Ukraine and elsewhere in Europe about the future of US aid, and Kyiv's ability to withstand Russian attacks in the absence of further American support.

Washington has provided more than $65 billion in military assistance to Ukraine since Russia launched its all-out invasion in February 2022, and a senior US defence official told journalists this week that a "substantial announcement" on aid is likely to come at the Thursday meeting.

As Russia's war approaches its third anniversary on February 22, Austin noted that the conflict has taken a significant toll on Moscow's interests, from its economy to its ability to project military power overseas.

With his forces tied down in Ukraine, President Vladimir Putin "didn't have the ability to respond to a challenge in Syria," said Austin, referring to the overthrow of Russian ally Bashar al-Assad by rebel forces last month.

"And as he loses his footprint in Syria, that takes away his projection platform -- he was using Syria to project combat power into Africa. Without that, his ability to continue to do that is now in question," the US defence chief said.