Keith Kellogg's last stop on his way to Kyiv, where he is due to arrive on Wednesday, was in European Union and NATO member Poland, which borders both Ukraine and Russia.

His visit came after top US officials met Russian negotiators in Saudi Arabia for the first time since US President Donald Trump blindsided allies by agreeing to launch talks with Russian counterpart Vladimir Putin.

"We have been assured... there is absolutely no American intention to reduce activity here in our part of Europe, especially in the area of security, (or) to reduce the number of American troops," Duda said after meeting with Kellogg.

Some European leaders, alarmed by Trump's overhaul of US policy on Russia, fear that Washington will make serious concessions to Moscow and re-write the continent's security arrangement in a Cold War-style deal.

On Friday, US Defense Secretary Pete Hegseth warned NATO allies in Europe against assuming that the American troop presence on the continent would "last forever".

But Duda asked Polish citizens "to remain calm" over US policy in the region.

"Once again, there is no fear that the United States will reduce its presence in our country," Duda, seen as a Trump ally, said.

Kellogg did not speak to reporters in the Presidential Palace in Warsaw.

He is set to arrive in Ukraine on Wednesday for three days of talks that will include a meeting with President Volodymyr Zelensky.