Russia's war in Ukraine in February has shifted political opinion in both Nordic countries in favour of joining the Western military alliance.

Membership requires consent of all 30 existing members but Turkey is putting a spanner in the works.

Erdogan has accused Sweden in particular of leniency towards Kurdish militants listed as a terror group by Turkey and the EU as well as members of US-based preacher Fethullah Gulen blamed by Ankara for an attempted 2016 coup.

"I spoke with the Dutch prime minister today," Erdogan said, adding he was due to hold telephone talks with the British and Finnish leaders on Saturday.

"We will also speak with (NATO chief Jens) Stoltenberg," he said. "We will continue to hold these talks so as not to cut the telephone diplomacy between us."

"We cannot say 'yes' to (Sweden and Finland) ... joining NATO, a security organisation," Erdogan said.

He said Turkey was a "victim" and had all the documents and information about "these terror organisations".

Erdogan this week had said that Turkey had requested the extradition of 30 "terrorists" from Sweden but this was turned down by Stockholm.

On Friday he said that if NATO was sensitive to Turkey's concerns, it should not extend membership to the two countries.

Sweden and Finland, while solidly Western, have historically kept a distance from NATO as part of longstanding policies aimed at avoiding angering Russia.

But the two nations both moved ahead amid shock over their giant neighbour's invasion of Ukraine, which had unsuccessfully sought to join NATO.

A delegation from Finland and Sweden had asked to visit Turkey on Monday but Erdogan turned down the request, saying "They shouldn't bother."

On Thursday, Stoltenberg said Turkey's "concerns" were being addressed to find "an agreement on how to move forward".