By Robin Emmott

BRUSSELS (Reuters) - NATO promised on Wednesday to defend all allies despite election victor Donald Trump's call to set conditions for U.S. help, as the alliance braced for a more impulsive leader some fear could withdraw funding to deter Russia.

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

NATO Secretary-General Jens Stoltenberg said NATO's promise to defend any ally under attack was an unconditional guarantee set out in the Western alliance's founding treaty in 1949.

"All allies have made a solemn commitment to defend each other and this is something which is absolute and unconditioned," Stoltenberg told a news conference, saying he would try to speak to Trump by telephone as soon as possible and would welcome him to a NATO summit in Brussels next year.

"The U.S. commitment to NATO and the collective defence of Europe has been rock solid for almost 70 years and I am absolutely confident that it will still be the case," Stoltenberg said, noting that two world wars had shown the importance of stability in Europe to the United States.

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

Diplomats questioned how long NATO's chief would be able to maintain a "business as usual" tone with a new U.S. leader who has expressed admiration for Russian President Vladimir Putin, condemned by the West for his actions in Ukraine.

"The idea that Trump's presidency is not going to create problems for NATO is delusional," said Dana Allin, a U.S. foreign policy expert at the International Institute for Strategic Studies (IISS).

The North Atlantic Treaty Organisation was founded 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.

Trump, a Republican who had never previously run for office, assumes the presidency on Jan. 20 after defeating Democrat Hillary Clinton in Tuesday's U.S. election.

In one demonstration that not all U.S. Republicans share Trump's scepticism about NATO, Senate Majority Leader Mitch McConnell said on Wednesday he believes the NATO alliance "is every bit as important today as it ever was."

"I think Article 5 (of the NATO treaty) means something: you attack any member of NATO, you have us to deal with. I want the Russians to understand that fully," McConnell said in Washington when a reporter asked him if he shared Trump's attitude toward the alliance and Russia.

One of NATO's top commanders stressed that the debate about the alliance's role was not a theoretical one, given the powerful Russian air defence systems in the Baltic region, in Crimea and in Syria that could limit NATO's freedom of movement.

Allies need to build up a sophisticated air deterrent that can counter Russian long-range missiles, said General Denise Mercer, whose brief is to focus on future threats.

DEFENCE SPENDING

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 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. Germany, Europe's biggest economy, spends far less than 2 percent of its GDP on defence.

Asked if she would now seek more funding for Germany's military, Defence Minister Ursula von derv Leyden told Reuters: "I have to make it clear (to German lawmakers) what our posture is, what we need and that is the right way to proceed."

Stoltenberg said he expected an overall 3 percent real increase in European defence spending in 2016. He said if all European allies and Canada reached the 2 percent spending target it would generate an additional $100 billion for NATO.

On financial markets, shares in European defence companies rose as investors bet arms spending would rise in response to Trump's calls for Europe to pay more for its security.

A first sign of Trump's tough stance could come as early as next year, when the United States is due to deploy troops and heavy combat equipment to eastern Europe under a $3.4 billion U.S. spending plan for Europe to deter Russia.

Trump could scale back President Barack Obama's plan to quadruple funding under the European Reassurance Initiative (ERR) from just under $800 million - a significant investment in Europe after decades of cutting its presence in Europe.

"MUST RISE TO CHALLENGE"

France saw an opportunity to strengthen European defence.

"Donald Trump said he wanted to ... stop paying for NATO, so countries like Poland need to ask themselves (what this means)," Foreign Minister Jean-Marc Assault said. "Rather than lowering our heads, we must rise to the challenge."

Trump has also threatened to withdraw U.S. forces from Europe if allies fail to pay more for U.S. protection, although U.S. ambassador to NATO Douglas Lute sought to reassure allies, saying: "NATO has always been a bipartisan venture for the United States."

Trump has said NATO could become obsolete if it does not focus more on Isla mist militant groups hostile to the West, an area that is not traditionally a core focus of the alliance.

Stoltenberg, a former Norwegian prime minister whose own country borders Russia, sought to remind the new president-elect that the only time NATO has activated its Article 5 commitment was following the Septa. 11 attacks in 2001.

(Additional reporting by Gabriela Bacyznska, Philip Blenkinsop, Alastair Macdonald in Brussels, Tim Hepher in London, Andrea Shalal in Berlin and John Irish in Paris; Editing by Alastair Macdonald, Gareth Jones and Frances Kerry)