The head of NATO labelled North Korea's behaviour "reckless" on Sunday and called for a concerted international response to the isolated state's recent nuclear and missile tests.

Pyongyang carried out its sixth and most powerful nuclear test to date on September 3.

"The behaviour of North Korea is a global threat and requires a global response, and that of course also includes NATO," Jens Stoltenberg, the military alliance's secretary-general, told the BBC.

NATO has not been directly involved in the crisis to date, but speculation has been mounting over whether the organisation's Article 5 - which says an attack on one member is an attack on all - would cover a North Korean offensive on the US Pacific territory of Guam.

Stoltenberg declined to clarify NATO's position on the matter, and instead stressed the need for a conflict-free resolution to the crisis on the Korean Peninsula.

"We are now totally focused on how we can contribute to a peaceful solution of the conflict," he said.

North Korea: Kim Jong-un praises nuclear test

Michael Fallon, the UK's defence minister, offered his support on Sunday for Stoltenberg's position and declared that the UK would continue to support fellow NATO member the United States.

"What we have to avoid at all costs is this spilling over into any kind of military conflict… It's United States sovereign territory [Guam] and the US, of course, under the United Nations, has the right to ask other members of the United Nations to join in its self-defence," Fallon told the BBC.

North Korea announced last month it was "carefully examining" a plan to attack Guam, which is 3,430km from North Korea and home to some 7,000 US troops.

Kim Jong-un, the leader of the isolated state, ordered the army to be ready to launch a missile strike on Guam on August 16.