Polish President Andrzej Duda on Thursday said the EU member would deliver an initial batch of four Soviet-designed MiG-29 fighter jets to Ukraine soon.

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

"Additional planes are currently in preparation, under maintenance, and will probably be transferred successively," he added.

"We can therefore say that we are on the verge of sending these MiGs to Ukraine," he said, adding that Poland currently has a dozen or so MiG planes that it inherited from the former German Democratic Republic.

"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," Duda said.

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

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 this coalition, Blaszczak mentioned Slovakia but added "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 since been made.

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

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

Blaszczak told Polish public radio that it was an "espionage group... collecting information for those who attacked Ukraine".