BRUSSELS — US Secretary of State Rex Tillerson will push his NATO allies to boost defence spending, at their first talks on Friday, by warning them that the disproportionate US share is "unsustainable", officials said.

Tillerson will be the latest top US administration official to deliver that message when he meets fellow NATO foreign ministers in Brussels.

The meeting is in preparation for a May 25 NATO summit in the Belgian capital which US President Donald Trump will attend.

While saying the allies themselves know they must do more, NATO chief Jens Stoltenberg dismissed concerns that Trump is less committed to the 28-nation alliance than his predecessors.

"They are very committed to NATO and the transatlantic bond because they see the importance of NATO for Europe but they also see the importance of NATO for the US," Stoltenberg told a press conference on the meeting’s eve.

Tillerson will also push the allies to increase their role in fighting terrorism, a senior State Department official told reporters ahead of the trip. 

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 had expressed fears Trump would 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 set off alarm bells in 2014 when it annexed Crimea from Ukraine and backed separatists in eastern Ukraine.

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

US officials said Tillerson will push the allies to do more to reach the goal of boosting defence spending to two percent of gross domestic product by 2024, which they set during a summit in Wales in 2014.

The NATO 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.

And Tillerson will point that out clearly on Friday.

"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. 

’National plannings’

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 US commitment to NATO even while demanding laggards in the alliance meet their spending pledges.

Stoltenberg, who heard similar messages during a recent visit to Washington, said the allies were looking at some "tools" to ensure increased spending.

In this way, he said, "all allies have to develop national plannings in which they outline how they can implement and reach the pledge they made in 2014 on defence investments."

The NATO meeting is due to begin at 10:50 (0850 GMT). — AFP