The US State Department confirmed that Washington had proposed new dates for a NATO meeting, after Tillerson decided to skip the original talks and rebuffed efforts to reschedule them.

The most senior US diplomat will be replaced by his deputy at the NATO meeting on April 5 and 6 in Brussels, while Tillerson remains at home to attend talks between US President Donald Trump and Chinese leader Xi Jinping. Tillerson is also due to visit Moscow in April.

The decision not to attend immediately led to new concerns about US President Donald Trump's loyalty to NATO, which Washington was quick to allay.

"The United States remains 100 percent committed to NATO. President Trump said this in his very first address to a joint session of Congress. He said our commitment to NATO is unwavering and it remains so," acting State Department spokesman Mark Toner said Tuesday.

Tillerson will meet European ministers

The State Department insisted Tillerson will meet most NATO foreign ministers at an anti-"Islamic State" (IS) conference later this week, adding that the decision was not unprecedented, and that former secretaries of state had missed the April meeting in 2003 and 1999.

The military alliance had offered to change the meeting dates so Tillerson could attend both it and the Xi talks, but the US State Department rebuffed the idea, a former U.S. official and a former NATO diplomat, both speaking on condition of anonymity, told the Reuters news agency on Monday.

Europe alarmed

The timing of the announcement has unnerved some European allies, particularly in the former Soviet bloc, who are dealing with a more assertive Russia.

During his election campaign, Trump described NATO as "obsolete" but has toned down his rhetoric since taking office in January, saying he strongly supports the alliance.

The snub came just a few days after Trump claimed on Twitter that Germany owes "vast sums of money to NATO and the United States," following his first meeting with German Chancellor Angela Merkel at the White House.

While NATO officials in Brussels on Tuesday played down the snub, former US diplomats expressed concern at the message Tillerson is sending.

"I think it is a mistake," Ivo Daalder, ex-US ambassador to NATO, told DW. "The idea that a secretary of state should skip a NATO ministerial (meeting) is remarkable, indeed it is almost unprecedented."

Another former ambassador to NATO, Harvard professor Nicholas Burns, told the Agence France-Presse news agency: "Of course Secretary Tillerson should be at the NATO meeting. We are the leader of NATO and should meet with allies before Russia."

Late on Wednesday, the White House said Trump would attend a separate meeting of NATO heads of state in Brussels in May. 

Tillerson moves on Montenegro

Meanwhile, Tillerson has written to US Senate leaders urging them to ratify Montenegro as NATO's newest member, saying it is "strongly in the interests of the United States."

The Reuters news agency cited a letter, dated March 7, arguing that the former Yugoslav state would support greater integration, security and stability among its neighbors.

Montenegro hopes to win the required approval of all 28 NATO allies in time to become a full member at a summit in May.

mm/gsw (AFP, AP, dpa, Reuters)