A chief adviser to Turkey's president told his U.S. counterpart that Turkey wanted "concrete steps" on the existence of what it calls "terrorist organisations" in Finland and Sweden before it would consider their NATO bids, the Turkish presidency said.

U.S. national security adviser Jake Sullivan and Ibrahim Kalin, chief foreign policy adviser to President Tayyip Erdogan, spoke on Monday to discuss the NATO bids and the war in Ukraine, the Turkish presidency said in a statement.

Turkey has objected to the two countries joining the Western defence alliance on the grounds that they harbour people linked to the Kurdistan Workers Party (PKK) militant group and others it deems terrorists, and because Finland and Sweden halted arms exports to Turkey in 2019.

In a statement, the Turkish presidency said Kalin had told Sullivan in a phone call that nations wanting to join NATO must "internalise the alliance's values and principles on security and counter-terrorism."

"It was emphasised that it is imperative for Sweden and Finland to take concrete steps regarding the terrorist organisations that threaten Turkey's national security," it added.

All 30 NATO members must approve plans to enlarge NATO.

Sweden and Finland have said they condemn terrorism and are open to dialogue.