NATO Secretary General Jens Stoltenberg has expressed confidence that Finland and Sweden will join the alliance, just days after Stockholm said it had done everything possible to allay Turkey's doubts about its membership.

The membership process usually takes years, he said, but in July all 30 members invited Finland and Sweden to join and signed accession protocols. Since then, 28 countries have approved the two countries' applications, with the exception of Turkey and Hungary.

This was the fastest accession process in modern NATO history, Stoltenberg said.

He also downplayed any risk that Finland and Sweden might be attacked or pressured into joining the alliance, saying the United States and other allies have offered the two countries security guarantees until they become full members.

It is inconceivable that Finland and Sweden would face any military threat without a NATO response, Stoltenberg added.

Last month, Turkish Foreign Minister Mevlut Cavusoglu said Sweden was not even halfway to fulfilling its obligations to Ankara. His statement came after a Swedish court ruled against the extradition of a journalist wanted by Turkish authorities on suspicion of involvement in a failed coup in 2016.

Swedish Prime Minister Ulf Kristersson said his country had fulfilled its obligations and the decision was now up to Turkey.