"We must remember that from the moment Sweden potentially applies, and NATO says that it wants Sweden to join, there is a very strong NATO obligation to guarantee Sweden's security", Stoltenberg told Swedish public television SVT.

"We have several ways of doing that, among other things with a heightened presence of NATO and NATO forces around Sweden and the Baltic Sea", he said.

In a historic turnaround sparked by Russia's invasion of Ukraine, Sweden and Finland -- two militarily non-aligned countries -- are considering applying for NATO membership.

A decision is expected in each country in the coming days or weeks.

An expansion of the Atlantic alliance is likely to anger Moscow, which has warned of "consequences" if the Nordic neighbours apply.

One of the most pressing questions is the two countries' security during an application process -- which can take months as each of the 30 NATO member states must ratify any membership bid.

Only full members benefit from NATO's Article 5, its cornerstone principle that an attack on one member is an attack on all members.

Stockholm and Helsinki have in recent weeks held talks with several major NATO powers to obtain "security assurances".

Finnish Prime Minister Sanna Marin on Thursday said discussions had already been held with the France, Germany, the UK and the United States, and stressed that a speedy ratification process would be "would be the best security guarantee".

Swedish Foreign Minister Ann Linde, who on Thursday wrapped up a visit to Canada and the United States, said Tuesday in Washington that the United States was prepared to provide "different kinds of security assurances" during the application process.

Asked by AFP, a spokesman for the US National Security Council said he was "confident" a solution could be found.