"I'm confident of course that Sweden will be a member, and then we're working for that to happen as early as possible," Stoltenberg said at a meeting of NATO foreign ministers in Oslo.

Stoltenberg said he had already spoken to Erdogan by telephone earlier this week to "highlight the importance of making progress" on Sweden's membership.

No date has been set for the visit, but it will take place at Erdogan's invitation, he added.

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

Finland formally joined the alliance in April.

Stoltenberg said he was "confident" that Hungary would also ratify Sweden's bid.

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

Swedish Foreign Minister Tobias Billstrom said Stockholm had fulfilled "all the commitments" to join NATO.

"It is time for Turkey and Hungary to start the ratification of the Swedish membership to NATO," he said.

"This was never a sprint, it's a marathon, and we now see the end of it."

Billstrom pointed to the entry into force on Thursday of new terror legislation in Sweden as the last step under an accord signed with Turkey last year.

Billstrom said he hoped to see a big step made towards membership at a meeting with representatives of Turkey in the coming weeks.

"Following that meeting, the ratification will happen," Billstrom insisted.

Many of the ministers present in Oslo said they wanted to see Sweden join before the NATO summit in Vilnius on July 11-12, which Stoltenberg said Tuesday was "absolutely possible".

"We're continuing to work to complete the accession process for Sweden, another very strong and capable partner, and we fully anticipate doing so by the time the leaders meet in Vilnius," US Secretary of State Antony Blinken said.

Support for NATO membership has reached record levels in the Scandinavian country, with 65 percent of Swedes in favour in May, according to a poll published on Thursday, compared to 60 percent in December.