"Yes I do," Trump said when a reporter asked if NATO countries should shoot down Russian aircraft if they enter their airspace.

His comments come after a series of recent incursions by Russian fighter jets and drones that have rattled Washington's NATO allies in Europe.

The US president meanwhile once again deflected questions about whether he believed Russian leader Vladimir Putin was ready to make peace despite recent Russian escalations.

"I'll let you know in about a month from now, okay?" Trump said when asked if he still trusted Putin, whom he met in a high-profile summit in Alaska in August.

The 79-year-old Republican has previously, and repeatedly, given deadlines of two weeks to make a decision on whether to take steps including fresh sanctions against Russia.

Trump, who had a televised bust-up with Zelensky in the Oval Office in February, added that he had "great respect for the fight that Ukraine is putting up. It's pretty amazing actually."

Zelensky thanked Trump for his "personal efforts to stop this war" and echoed Trump's call for European countries to stop buying Russian oil.

Tensions between Russia and Europe over Ukraine have escalated with the recent spate of aerial violations.

NATO scrambled jets after three Russian MiG-31 fighters on Friday breached Estonian airspace for some 12 minutes, prompting Estonia to call for a meeting of the UN Security Council and talks with NATO allies.

Fellow NATO member Poland said earlier this month that Russian drones had repeatedly violated its airspace during an attack on Ukraine, in what Warsaw called an "act of aggression."

Trump has previously said the Polish incident could have been a mistake, but his latest comment represents a signficant hardening of his position.