NATO Secretary General Jens Stoltenberg on Monday said Turkey should approve Sweden's stalled bid for membership "as soon as possible".

"Sweden has delivered on what they promised and now the time has come for Turkey to finalise the accession process," Stoltenberg said.

Turkey and Hungary are the only NATO members to not yet have ratified Sweden joining the alliance more than 18 months after it applied for membership.

The Turkish parliament started this month to debate Sweden's bid to join after President Recep Tayyip Erdogan launched the process following a deal at a NATO summit in July.

NATO's other 29 allies had hoped to be able to formally welcome Sweden into the alliance at a meeting of foreign ministers in Brussels this week.

But the process is still at the committee level in the Turkish parliament.

"I would have liked to see more speed in the ratification process, that's no secret," Stoltenberg said.

"I would like them to finalise that, and that's exactly what I have communicated many times."