Stoltenberg said he had discussed preparations for the meeting of heads of state and government with British Prime Minister Theresa May during a visit to London last week.

The December summit will be a chance to "address current and emerging security challenges and how NATO continues to invest and adapt to ensure it will remain a pillar of stability in the years ahead," Stoltenberg said in a statement.

He added that London was a fitting venue to mark 70 years of transatlantic military cooperation as it was home to the alliance's first headquarters after the United Kingdom become one of NATO's 12 founding members in 1949.

Nowadays the there are 29 member states and the headquarters is in Brussels.

"London was the home of our first headquarters, so it is a fitting venue for NATO heads of state and government to plan the Alliance's future," said Stoltenberg.

- Delicate time for NATO -

Last month NATO marked the 70th anniversary of the signing of its founding treaty with a meeting of alliance foreign ministers in Washington.

The meeting closed with a joint statement pledging new action to counter Russia's "aggression", with tensions with Moscow at a high not seen since the Cold War.

The alliance has approved new surveillance measures and naval exercises in the Black Sea in support of Ukraine and Georgia, aspiring members of NATO facing Russian-backed separatist forces.

And US Secretary of State Mike Pompeo said NATO had agreed to study strategies to counter non-traditional warfare from Russia, which is accused of meddling in a series of Western elections.

But the festive mood of the anniversary year has been clouded by lingering concerns about US President Donald Trump's commitment to the alliance and his willingness to honour its mutual self-defence pact.

Trump has been blunt in his criticism of NATO's European members, accusing them of freeloading on the protection offered by the US military while not spending enough on their own armed forces.

Before taking office Trump called NATO "obsolete", though he has also welcomed plans for allies to increase defence spending.

NATO summits normally conclude with a formal, binding statement of aims and actions agreed by all allies -- such as the 2014 agreement to try to spend two percent of GDP on defence.

It is yet to be confirmed whether a statement will be issued at December's meeting.

Britain is due to leave the European Union in October and the December summit will be seen as a signal of solidarity between NATO and the UK -- which is the continent's leading military power, along with France.

"Brexit will change the United Kingdom's relationship to the European Union but it will not change the United Kingdom's relationship to NATO," Stoltenberg said in February.

On Wednesday, he was back in London for talks with British Defence Secretary Penny Mordaunt. On Thursday, he will participate in a conference on cybersecurity in the British capital.