Norway hosts NATO's biggest exercises since the end of the Cold War from Thursday, to remind Russia the Alliance stands united despite seeds of doubt planted by US President Donald Trump.

Russian intelligence was spying on former double agent Sergei Skripal and his daughter Yulia for at least five years before they were poisoned in a nerve agent attack, Britain's National Security Adviser Mark Sedwill said in a letter to NATO on Friday.

NATO chief Jens Stoltenberg on Tuesday warned the EU its new defence cooperation pact must not undermine the alliance, as US officials voiced fresh concerns about the flagship initiative.

The head of NATO on Monday praised embattled US Secretary of State Rex Tillerson for his "key role" in the North Korea crisis as rumours swirl that his position is under threat.

NATO Secretary General Jens Stoltenberg said Sunday that North Korea's nuclear and missile programme represented a "global threat and requires a global response".

NATO allies will increase troop numbers in Afghanistan to help the government battle a resurgent Taliban but there is no question of returning to a combat role, the alliance's chief Jens Stoltenberg said Thursday.

NATO has begun training Iraqi soldiers on how to neutralise bombs planted by the Islamic State jihadist group, the alliance said Sunday, expanding a programme already in place in neighbouring Jordan.

Completion of NATO's new $1.2-billion headquarters in Brussels, expected to open this year, has been held up by problems with its IT and communications systems, Belgium said on Monday.

NATO chief Jens Stoltenberg Sunday defended the alliance's decision to refrain from stepping into the war in Syria, saying doing so would only make matters worse.

Damaging economic sanctions imposed on Russia over the Ukraine crisis must be kept in place to force Moscow to meet its Minsk ceasefire commitments, NATO head Jens Stoltenberg said Wednesday.

U.S. Secretary of State, John Kerry, appearing at his last NATO ministerial meeting, agreed that “unity is very, very important” for the defence alliance.

NATO head Jens Stoltenberg on Tuesday said he was sure that Donald Trump would meet all US commitments to the alliance, just days after urging the president-elect not to go it alone.