"I think this is about expectations management and to place this in its historical context," said a high-ranking German government source.

"And given the historical dimension, it would not be a catastrophe if we need a few more weeks" to find a deal, the source added.

"What is crucial is that in our view there are no insurmountable difficulties" between Sweden, Finland and Turkey, said the source.

"We are confident, very confident that a solution can be found that takes into account both sides' interests."

NATO had originally expected Finland and Sweden's membership bids to sail through quickly at a time when the alliance is keen to show an united front vis-a-vis Russia.

But Ankara's last minute resistance has thrown them off course.

Ankara has accused Finland and Sweden of providing a safe haven for the outlawed Kurdistan Workers' Party (PKK), listed as a "terrorist" group by Turkey and its Western allies.

Ankara has also demanded that they lift their weapons freezes on Turkey.

Any NATO membership deal must be unanimously approved by all 30 members of the alliance, and fears are now mounting that Turkey could delay the Nordics' bids indefinitely.