Ukraine has repeatedly asked its Western allies to send warplanes -- although seeking primarily modern US-made F-16s -- but so far no Western countries had committed to give jets to Kyiv.

"In the coming days we will first transfer, if I remember correctly, four fully operational planes to Ukraine," Duda told reporters of the Soviet-designed jets.

Sending any fighter jets would mark a crucial development in the conflict and would boost Kyiv's defence against Russian missile strikes and drone attacks.

Poland has been spearheading efforts to send more equipment to Ukraine and convince the West to follow suit.

Duda said Poland currently has a dozen or so MiG planes that it inherited from the former German Democratic Republic, which they are "on the verge" of sending to Ukraine.

"These MiGs are still in service in Poland's air force. They're in their last years of operation but are still for the most part in full working order," he said.

He also added that MiGs are "jets that Ukrainian pilots can operate today without any additional training."

A Ukrainian military spokesman said the country was "grateful to our partners".

"MiGs will not solve the tasks, we need F-16s. But MiGs will help to strengthen our capabilities indeed," Ukrainian Air Force spokesman Yuriy Ignat said.

- Slovakia also mulling planes -

There was no immediate comment from Moscow on the Polish decision. But the Kremlin has repeatedly said that Western arms supplies to Ukraine will only prolong the conflict, increase Ukrainian civilian suffering and have no impact on Russia's military aims.

Poland's move to become the first country to send MiG-29 warplanes to Ukraine does "not change" the US decision against sending its own warplanes to Kyiv, the White House said.

"It doesn't change our calculus with regards to F16s," National Security Council spokesman John Kirby told reporters, referring to the US-built fighter jet.

Earlier Thursday, Polish Defence Minister Mariusz Blaszczak had said Poland wanted to transfer MiG-29 planes to Ukraine "within the framework of a larger coalition of countries".

Asked about the countries in the coalition, Blaszczak mentioned Slovakia but added that "of course we are open to others."

"We absolutely want to conduct our activity within the framework of a coalition," he added.

Last year, Poland's neighbour Slovakia had said it was willing to discuss sending MiG-29 planes to help replace losses to Ukraine's current stocks.

However no final decisions have been taken.

Duda made the Polish announcement following talks with visiting Czech counterpart Petr Pavel, a former NATO general.

The MiGs transferred by Warsaw to Ukraine will be replaced at home with South Korean FA-50 planes bought by Poland, followed by American F-35s.

The president's comments also came after his defence minister had earlier said Polish counterintelligence had dismantled a Russian spy ring.