Poland, a NATO member and a staunch supporter of Ukraine, shares a border with both Kaliningrad exclave and Belarus, Moscow's ally.

"If our allies decide to deploy nuclear arms on our territory as part of nuclear sharing to reinforce NATO's eastern flank, we are ready to do so," Duda said in an interview published by the Fakt daily.

Moscow in response warned it would take steps to "ensure its security".

Duda spoke to the Polish media after a visit to New York, where he held meetings at the UN and discussed the war in Ukraine with former US president Donald Trump.

In March, he visited Washington DC, where he met with US President Joe Biden.

Discussions about nuclear cooperation between Poland and the United States have been going on "for some time", he said.

"I have already talked about this several times. I must admit that when asked about it, I declared our readiness," Duda said.

"Russia is increasingly militarising Kaliningrad. Recently it has been relocating its nuclear weapons to Belarus," he added.

- 'Response steps' -

Speaking to reporters later on Monday, Polish Prime Minister Donald Tusk said he "would have to understand the president's intentions," suggesting the matter had not been discussed between them.

"I would have to know all the circumstances that prompted the president to make this declaration," Tusk said.

"I'm looking forward to meeting President Duda, because the matter directly and very clearly concerns Polish security," Tusk added.

He also said he would host NATO chief Jens Stoltenberg and Britain's Prime Minister Rishi Sunak in Warsaw on Tuesday.

Pro-EU coalition leader Tusk has frequently clashed with the right-wing Polish president, but their views on supporting Ukraine and defying threats from Russia have so far been largely similar.

The Kremlin said it would respond if Poland hosted nuclear weapons.

"The military will of course analyse the situation and in any case will take all necessary response steps in order to ensure our security," Kremlin spokesman Dmitry Peskov told reporters.

In June 2023, Russian President Vladimir Putin confirmed that Russia had sent tactical nuclear arms to Belarus, which borders Ukraine and Poland.

During the last NATO summit in Vilnius, the allies pledged to "take all necessary steps to ensure the credibility, effectiveness, safety and security of the nuclear deterrent mission."