"There are no guarantees but it's absolutely possible to reach a solution and enable the decision on full membership for Sweden by then," Secretary General Jens Stoltenberg told reporters in Oslo on the eve of a NATO foreign ministers' meeting in the Norwegian capital.

"We don't have any certainty. Of course we are speaking about sovereign decisions by national parliaments," he said, but "my message is that it is within reach."

Stoltenberg said there was "a window now especially after the Turkish elections and with a Turkish parliament being constituted."

"Of course it is possible and we are working hard for this to happen as soon as possible."

Turkey and Hungary are the only NATO countries that have yet to ratify Sweden's membership bid.

Finland formally joined the alliance in April.

Turkish President Recep Tayyip Erdogan, who was re-elected Sunday for another five-year term, has accused Sweden of being a haven for "terrorists", especially members of the Kurdistan Workers' Party (PKK).

Stoltenberg said he was in "constant contact" with Turkish authorities to try to get the final obstacles to Sweden's accession lifted.