Nato's general secretary, Jens Stoltenberg, has said North Korea's nuclear programme "is a global threat and requires a global response".
"That of course also includes Nato," he told the BBC's Andrew Marr Show.
But he refused to say whether an attack on the US overseas territory of Guam would trigger the military alliance's collective defence pact.
Article 5 of the Nato treaty says that an attack against one member is considered an attack against all.
Guam, an island territory of the US in the Pacific Ocean, is a key military outpost and has been the subject of heated rhetoric from Pyongyang.
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"I will not speculate about whether Article 5 will be applied in such a situation," Mr Stoltenberg said.