The statement comes after a NATO summit earlier this month in Britain exposed sharp divisions between the leaders of its member states.

"We were just in London marking the 70th anniversary of our NATO alliance," said Stoltenberg, speaking in Kuwait to mark 15 years of the Istanbul Cooperation Initiative.

The ICI was launched by NATO to boost security links with the Middle East.

"In all these years, NATO has never experienced a more complex security environment than it does today," the NATO chief added.

"To maintain our security, we need to be agile and prepared to respond to threats coming from all directions -- from land, sea, air, space and cyberspace -- also from state and non-state actors."

NATO's 29 members vowed earlier this month to stick together against threats from Russia and terrorism and recognised the challenge of a rising China at the two-day meeting in Britain.

But leaders clashed over personality and policy, with US President Donald Trump branding Canada's Prime Minister Justin Trudeau "two-faced" after a group of allied leaders were caught on video at a Buckingham Palace reception mocking the US leader's rambling press appearances.

The run-up to the summit was also marked by France's President Emmanuel Macron branding the alliance "brain dead" and demanding a new strategy, reopening a dialogue with Russia and refocusing on the fight against Islamist terrorism.

Despite the rancour, the 29 leaders managed to agree a "London Declaration" which was the first from NATO to acknowledge the growing strategic challenge posed by China.

It also stressed the need for a stronger coordinated response against terrorism.

"We must continue to continue to fight terrorism. NATO plays a key role in this fight, working with our partners around the world to train local forces," said Stoltenberg on Monday.

"All NATO allies as well as NATO itself are part of the US-led global coalition against Daesh," he added, using the Arabic acronym for the so-called Islamic State jihadist group.