The former Norwegian prime minister, who took the helm in October 2014, will lead the alliance into its next summit in Brussels in July with the North Korean nuclear crisis and US President Donald Trump's apparent ambivalence towards NATO looming large.

The alliance is making ambitious reforms to its command structures that aim to make NATO fit for the challenges of warfare in the 21st century -- particularly the threat from cyber tactics and hybrid warfare.

Former Norwegian prime minister Stoltenberg, 58, wrote on Twitter that he was "honoured and grateful" to have his mandate extended.

"Looking forward to continue working together to strengthen the Alliance and keep our citizens safe," Stoltenberg wrote.

The North Atlantic Council, NATO's main political decision-making body, said the 29 alliance members had agreed to extend Stoltenberg's mandate to September 30, 2020.

Stoltenberg is the first NATO chief since the end of the Cold War to be given a two-year extension rather than just one -- an indication of confidence in his leadership.

And if he completes his term, Stoltenberg will become the longest-serving NATO chief since the fall of communism.

"Allies congratulate the secretary general and have full confidence in his ability to continue his dedicated work to advance NATO's adaptation to the security challenges of the 21st century," the council said in a statement.

- May, Macron congratulations -

British Prime Minister Theresa May welcomed Stoltenberg's reappointment, saying he had been a "true champion" of NATO.

"He has made sure that NATO has stood strong but not stood still, meeting Russian aggression in eastern Europe while reforming to face developing threats such as cyber attacks and hybrid warfare," May said in a statement.

French President Emmanuel Macron tweeted his congratulations, saying he had "full confidence in (Stoltenberg's) determination to pursue the adaptation" of the alliance.

Stoltenberg took charge of NATO as the Ukraine crisis raged in the wake of Russia's annexation of Crimea, and he has overseen the deployment of four combat-ready battle groups to the Baltic states and Poland to counter Russian assertiveness.

Some 4,000 troops have been posted, along with tanks and equipment, in NATO's biggest European deployment since the end of the Cold War which aims to let the Kremlin know NATO is ready to defend its members in the event of attack.

Last month NATO defence ministers agreed to increase the use of cyber weaponry and tactics during military operations and to create a cyber operations hub as the alliance faces hundreds of attacks on its networks every month and fears grow over the Kremlin's electronic tactics.

Stoltenberg said NATO's success since its creation in 1949 was founded on its ability to adapt to changing security challenges and he vowed to press ahead with his reforms.

"Adaptation will never be completed. The only way to successful adaptation is to understand that it's something that has to go on -- continuous adaption is key," he told reporters after his reappointment.

"NATO is a tool and a tool has to be sharpened and improved all the time to be able to tackle the challenges of the day."

Next year's summit is also set to forge ahead with changes to NATO's command structure, stripped back after the Cold War but now being expanded with the revival of a centre overseeing Atlantic shipping lanes.

The alliance is also keen to further boost cooperation with the European Union -- already significantly stepped up under Stoltenberg -- to make it easier to move troops and equipment around Europe.

The growing nuclear threat from North Korea also casts a shadow over the alliance, after Pyongyang tested a ballistic missile earlier this month which it said brought the whole of the continental United States within range.

source: AFP