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NATO's secretary general says the military alliance 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.

Secretary General Jens Stoltenberg said during a speech at a security conference in Sweden on Sunday that ridding the world of nuclear weapons requires a period of "painful disarmament" that the U.N. convention cannot guarantee.

Stoltenberg's message was clearly directed to the Swedish government, which is divided on whether to sign the convention.

The NATO chief told Swedish media earlier that Sweden, which is not a NATO member, could find its cooperative relationship with the alliance weakened if it does.

Pope Francis last week called on nations to work toward a binding nuclear weapons ban.