STOCKHOLM -- NATO supports the idea of a world without nuclear weapons, but doesn't believe it can be achieved by imposing a ban through the United Nations convention on nuclear weapons, the military alliance's top official said Sunday.

Secretary General Jens Stoltenberg said during a speech at a security conference in Sweden that ridding the world of nuclear weapons requires a period of "painful disarmament" that the Treaty on the Prohibition of Nuclear Weapons cannot guarantee.

Stoltenberg stressed that NATO's military deterrence strategy relies on a combination of conventional weapons and nuclear weapons. If NATO members scrapped their nuclear arsenals but countries such as China and Russia kept theirs, the world would not be made safer, he said.

Stoltenberg's message was clearly directed to the Swedish government, which is divided on whether to sign the U.N. treaty. Swedish Foreign Minister Margot Wallstrom is among those supporting it. The government is unlikely to decide before the next parliamentary election in September.

The NATO chief told Swedish daily newspaper Dagens Nyheter earlier that Sweden, which is not a NATO member, could find its co-operative relationship with the alliance weakened if it endorses the U.N. convention.

The International Campaign to Abolish Nuclear Weapons, or ICAN, has been working to promote the U.N. convention. The NGO was awarded the 2017 Nobel Peace Prize for its efforts to draw attention to the humanitarian consequences of nuclear weapons and to achieve a treaty-based prohibition of such weapons.

Pope Francis last week called on nations to work toward a binding nuclear weapons ban, and voiced his concern over the tensions on the Korean peninsula. The Holy See was among the 122 nations that approved the treaty last year.