Stoltenberg stressed that NATO allies had provided key assurances to protect Sweden in the interim period before it becomes a full member and can benefit from the alliance's Article 5 mutual defence agreement.

"Seen from a security perspective, Sweden is in a better place now than before it applied", he said, adding: "NATO allies responded by... issuing security assurances to Sweden", citing the US and Britain in particular.

He said that if the Scandinavian country were attacked, it was "unthinkable that NATO allies would not react. That is a message that NATO allies have conveyed in a very clear way to any potential adversary".

Speaking at a joint press conference with Swedish Prime Minister Magdalena Andersson at the prime minister's summer residence Harpsund, Stoltenberg also said NATO was working "hard and actively" together with Stockholm and Ankara to resolve Turkey's concerns "as soon as possible".

Ankara accuses Sweden and to some degree Finland, which have submitted joint NATO bids, of providing a safe haven for the outlawed Kurdistan Workers' Party (PKK), listed as a "terrorist" group by Turkey and its Western allies.

Ankara is also angered by Sweden's embargo on arms sales to Turkey since 2019 over Ankara's military offensive in Syria.

Any NATO membership deal must be unanimously approved by all 30 members of the alliance.

"We take the Turkish concerns very seriously, not least the security concerns when it comes to the fight against terrorism," Swedish Prime Minister Andersson said.

She noted that tougher anti-terrorism legislation was scheduled to come into force in Sweden on July 1, and said Sweden's independent weapons export agency would be prepared to review its policy once the country was a NATO member.

Stoltenberg had previously said Sweden and Finland would be welcomed into NATO "with open arms", and expected the Turkey issue to be resolved before a NATO summit due to begin in Madrid on June 28.

But speaking in Finland on Sunday he suggested the dispute may drag on, saying "the summit in Madrid was never a deadline".