US Secretary of State Rex Tillerson headed to Brussels on Friday to push President Donald Trumps demand for NATO allies to boost defence spending.

Tillerson is meeting fellow NATO foreign ministers for the first time, ahead of a May 25 leaders summit with Trump in Brussels.

Seeking to draw a line under the funding row, NATO chief Jens Stoltenberg has sought to dismiss concern that Trump is less committed to the 28-nation alliance than his predecessors.

"We see a strong US commitment to NATO, to the transatlantic bond not only in words but also in deeds," Stoltenberg told reporters shortly before the talks start, citing increased US military deployments in eastern Europe.

He said the European allies themselves realised the need to meet pledges made in 2014 to increase defence spending to two percent of gross domestic product by 2014, citing conflicts to the east and south of its borders.

"But it is not just about cash. It is also about investing in the capabilities we need and committing forces to NATO deployments," Stoltenberg said.

Tillerson, due in Brussels at 10:45 (0845 GMT), will also push the allies to increase their role in fighting terrorism, US officials said.

Tillerson travelled to Brussels following a visit to Turkey, a NATO ally and key player in both Syria and Iraq where Washington wants to defeat the Islamic State group.

In the last two years, IS has claimed or hailed a wave of deadly attacks in US and European cities, including Brussels.

Tillerson will also work with NATO allies to press Russia to fulfil its obligations under the Minsk agreements to end the war in eastern Ukraine, the US official said.

Allies have been alarmed at the prospect of Trump seeking to improve relations with Russia at the expense of support for the pro-Western government in Ukraine or NATO allies in former Soviet parts of eastern Europe.

Their concerns were reinforced when Tillerson initially planned to skip the NATO meeting, citing various commitments including a trip to Russia.

But Tillerson, a press-shy former oilman who had friendly ties with Russian President Vladimir Putin, agreed to attend when NATO rescheduled the meeting for Friday.

Russia in 2014 annexed Crimea from Ukraine and backed separatists in eastern Ukraine.

A senior NATO official told reporters on condition of anonymity that the Trump administration was now taking a more "mainstream" approach to the alliance and anxiety among allies had eased.

Two percent target remains contentious

But the two percent of GDP funding aim by 2024 remains central to the future of NATO.

Its 2016 annual report said only five countries met the two percent target -- the United States, Britain, Greece, Poland and Estonia -- while Washington still accounted for nearly 70 percent of combined alliance defence spending.

Tillerson will point that out clearly on Friday.

jpegMpeg4-1280x720"It's no longer sustainable for the United States to maintain a disproportionate share of NATO's deterrence and defence spending," the State Department official said.

Tillerson will also push very hard for allies to spend a fifth of their defence budgets on military-capacity building, the US official said.

But the official gave no indication of what would happen if the allies fail to deliver on increased spending.

During a visit to NATO in February, US Defense Secretary James Mattis voiced staunch support for NATO but warned that Washington could "moderate" its commitment if allies fail to pay up.

Making a similar visit the same month, US Vice President Mike Pence also stressed the US commitment to NATO even while demanding laggards in the alliance meet their spending pledges.

Stoltenberg heard similar messages during a recent visit to Washington.

"I expect national plans to be an important topic to be discussed today," he said, alluding to plans to implement spending pledges.