WASHINGTON (AFP) - US Secretary of State Rex Tillerson will present a meeting of the Nato allies on Friday (March 31) with a demand from President Donald Trump that they increase defence spending.

The annual Nato foreign ministers talks in Brussels were brought forward at the last minute after Mr Tillerson warned he would not be able to attend on the long-planned date.

Washington's top diplomat is reportedly keeping time free in early April to take part in a possible golf resort summit between Mr Trump and China's leader Xi Jinping.

But Mr Tillerson needs to meet with his colleagues to prepare for Mr Trump's first full Nato summit on May 25.

And his apparent reluctance to commit to the Nato meeting only served to reinforce the impression that Mr Trump places little stock in America's decades-old alliances.

This month, Mr Trump's conviction that the allies must somehow pay Washington for the reassuring presence of US forces in Europe cast a pall over talks with German Chancellor Angela Merkel.

"Germany owes vast sums of money to Nato and the United States must be paid more for the powerful, and very expensive, defence it provides to Germany!" he tweeted.

In barnstorming speeches on the campaign trail last year, Mr Trump declared Nato to be obsolete. But since coming to office his administration has taken a more positive stance.

Acting as the good cops while bad cop Mr Trump lays out cash demands, Vice-President Mike Pence and Defence Secretary Jim Mattis have reaffirmed the US commitment to Nato.

Mr Tillerson, having finally agreed to meet his colleagues at his first Nato talks, must now translate Mr Trump's tweets into a diplomatic strategy to strengthen the alliance.

'HARD PUSH'

The press-shy former oilman has kept his thoughts to himself but State Department officials - speaking on condition of anonymity - briefed reporters.

"The first goal that Secretary Tillerson's going to push is to get the allies to renew their commitment through increased resources for Nato defence spending," one said.

"It's essential that the allies honour their commitment from the last two previous summits to spend two per cent of their gross domestic product on defence," he explained.

In addition to the two per cent goal, the official said, Mr Tillerson will push very hard for allies to spend a fifth of their defence budgets on military-capacity building.

The 28-member alliance met in Wales in 2014 and agreed that each would boost defence spending to the two per cent goal by 2024 - leaving them today with a seven-year grace period.

But Mr Trump has made it clear he regards previous shortfalls in this goal - even before the Wales deal - as representing a growing debt from member states towards the alliance.

Some Nato members, particularly those on the alliance's eastern flank exposed to potential Russian interference, have met or are close to meeting the goal.

But Mr Trump wants Nato to refocus its efforts away from deterring Vladimir Putin's Russia and towards his global goal of eradicating "radical Islamic terrorism."

"Secretary Tillerson will be pushing allies in that way as well," a senior State Department official said.

The US defence budget is 68 per cent of Nato members' combined spending, but previous presidents have assumed the burden as part of a global leadership role.

Russia meanwhile has taken increasingly provocative steps in Ukraine and Syria, just across Nato's eastern and southern borders, spooking many of the allies.

Allied warplanes regularly intercept Russian bombers over the North Sea and Russia is accused of covertly backing anti-EU and anti-Nato populist movements across Europe.

UNSUSTAINABLE SPENDING

Washington still insists it has the allies' back - but apparently not at any price.

"The President has made very clear, and the Secretary will reinforce this on Friday," a senior US official said.

"It's no longer sustainable for the United States to maintain a disproportionate share of Nato's deterrence and defence spending."

On Thursday, Mr Tillerson will be in Ankara, meeting senior leaders of Nato ally Turkey, which has increasingly strained relations with European capitals but a key role in Syria.

While he is there, Nato ambassadors will meet their Russian counterpart as part of the Nato-Russia council, a forum which has been mothballed since Moscow's 2014 intervention in Ukraine.

On Friday, Mr Tillerson - who had such friendly ties with Putin as CEO of energy giant ExxonMobil that he won a Kremlin medal - will meet with his Ukrainian counterpart.

And he will lend American support to Nato's efforts "to push Russia to end its aggression" in Ukraine.