If no agreement is found soon, the meeting will go ahead as scheduled on April 5-6, three diplomats said. There has also been discussion about a gathering before the G7 foreign ministers meeting in Italy on April 10-11, which Tillerson will attend, but March 31 is the preferred date, diplomats said.

A NATO official said there was no decision yet. NATO Secretary-General Jens Stoltenberg told Reuters in an interview this week that both NATO and the United States were looking for a new date for talks.

Reuters exclusively reported on Monday that Tillerson, a former top executive at Exxon Mobil Corp who worked with the Russian government, would skip his first meeting with all NATO foreign ministers for a US visit by the Chinese president and travel to Russia in April.

Tillerson's potential no-show increased unease among European allies about US President Donald Trump's commitment to the Western military alliance despite assurances by his defence minister and vice president of support.

During his election campaign and on the eve of taking office, Trump called NATO "obsolete". He has since said he strongly supports the alliance, but in interviews and speeches he continues to air grievances over what he see as Europe's failure to pay its fair share of protecting the West.

Tillerson met many of the NATO foreign ministers in Washington this week at a gathering of the coalition fighting Islamic State militants, but the meeting in Brussels would be his first formal NATO ministerial.

Given the US role as the de facto head of the alliance, it is rare for the United States' top diplomat to miss a NATO meeting. The last time was during the Iraq war in 2003, when Colin Powell was forced to cancel at the last moment.