BRUSSELS: NATO's promise to defend and protect any ally under attack is an unconditional guarantee set out in the Western alliance's founding treaty, the organisation's chief said on Wednesday following the victory of Donald Trump in the U.S. election.

During the campaign, Trump threatened to abandon U.S. allies in Europe if they do not spend enough on defence, unnerving the ex-Soviet Baltic states on Russia's border, who fear Moscow might try a repeat of its 2014 annexation of Ukraine's Crimea.

"NATO's security guarantee is a treaty commitment and all allies have made a solemn commitment to defend each other and this is something which is absolute and unconditioned," NATO Secretary-General Jens Stoltenberg told a news conference.

Trump's suggestion of conditioning the United States’ defence of its Western allies was the first time a leading presidential candidate had raised the idea, putting him directly at odds with NATO's 27 other member states.

The North Atlantic Treaty Organisation was founded in 1949 around the central promise that an armed attack against one ally is an attack against all, whether it be on land, in the air or on the seas. NATO leaders in July added the area of cyberspace to that list of war fighting domains, although allies have to request NATO's help and decisions are on a case-by-case basis.

After the fall of the Soviet Union a quarter of a century ago, NATO's European allies cut defence spending to historic lows, leaving the United States to make up around three quarters of the alliance's military expenditure.

A newly assertive Russia under President Vladimir Putin has begun to change that and Europe is again spending more on defence. But Britain, Poland, Greece and Estonia are the only European nations to meet a NATO goal of spending at least 2 percent of gross domestic product on defence.

Trump also threatened to withdraw U.S. forces from Europe if allies fail to pay more for U.S. protection.

Stoltenberg, a former prime minister whose own country Norway borders Russia, sought to remind the new president-elect that the only time NATO had activated its so-called Article 5 commitment, was in the defence of the United States -- following the Sept. 11 attacks on New York and Washington in 2001.

He also said NATO allies were a big part of the U.S.-led strategy to stabilise Afghanistan and rid it of militants hostile to the West, with a long-standing NATO presence in the country since the 2001 attacks.

NATO "is important both for collective defence in Europe and to provide help and play a role in the fight against international terrorism", Stoltenberg said.

Addressing European diplomats at the U.S. embassy in Brussels, the U.S. ambassador to NATO, Douglas Lute, sought to reassure allies of continuing commitments from Washington:

"There's a lot of continuity here," he said. "NATO has always been a bipartisan venture for the United States."