US Secretary of State Rex Tillerson will skip a NATO meeting in April but travel to Russia the same month, officials said Tuesday, fuelling fears about Washington's commitment to the alliance.

Tillerson will be replaced by his deputy at the Brussels meeting on April 5 and 6, despite Washington's efforts to quash questions about US President Donald Trump's support for NATO and quest for better ties with Moscow.

Chinese leader Xi Jinping is expected to visit Trump at his Florida golf resort at some point in early April, and Tillerson would be expected to attend, but State Department officials gave no public explanation for his decision to skip Brussels.

Instead, they insisted Tillerson will in any case meet most of the foreign ministers from the 28-nation military alliance at the State Department in Washington this week at a meeting of the coalition fighting the Islamic State (IS) militant group.

"He has already met with officials from Ukraine. After these consultations and meetings, in April he will travel to a meeting of the G7 in Italy and then on to meetings in Russia," one senior US official said.

In Brussels, the United States will be represented by Tom Shannon, the highest ranking career diplomat left in the State Department from the previous administration and Tillerson's acting deputy.

After almost two months in the job, Tillerson has yet to appoint a deputy or any assistant secretaries and he has adopted a low-profile stance, avoiding public and press events and working with a small inner circle of advisers.

The administration, meanwhile, has been scrambling to reaffirm its commitment Washington's military alliances -- including NATO -- after Trump called into question their usefulness during his campaign.

Just last week -- after meeting Chancellor Angela Merkel at the White House -- Trump took to Twitter to claim that Germany owes "vast sums of money to NATO and the United States," reviving his charge that US allies are not paying their way.

Trump's Defense Secretary James Mattis, a former Marine general, has declared that the United States fully supports NATO, and Tillerson travelled last week to reaffirm ties with Asian allies Japan and South Korea.

But the United States is by far the North Atlantic alliance's leading partner, and Tillerson's absence from its foreign ministers' meeting will be noted with concern, especially by newer East European members on its exposed east flank.

NATO Secretary General Jens Stoltenberg was in Washington on Tuesday for meetings at the Pentagon ahead of Wednesday's coalition talks, and NATO officials played down the apparent Tillerson snub.

"All allies are represented at NATO ministerial meetings, which are important regular events. It's up to allies to decide at what level they are represented," one said, speaking on condition of anonymity.

"The secretary general will continue his regular contacts with the US administration, which has confirmed its strong commitment to NATO, both in words and in deeds."

But former US officials expressed concern at the message Tillerson is sending.

Ivo Daalder, a former US ambassador to NATO and now president of the Chicago Council think tank, said on Twitter that the decision "feeds growing allied doubts about US commitments."

And he dismissed the State Department's attempt to build up this week's anti-IS meeting as an alternative venue, saying the April foreign ministers' meeting is crucial preparation for the full NATO summit that Washington "can't miss."

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

Under Obama, the United States worked with NATO to shore up support for the pro-western government in Kiev after Russia's annexation of Crimea and its support for a bloody uprising in eastern Ukraine.

Combined with economic sanctions, the deployment of more NATO troops from Western members to frontline Eastern allies in the Baltics and Poland was intended to send a signal to Moscow that further intervention would not be tolerated.

But during his successful 2016 presidential campaign, Trump struck a more emollient tone with Moscow, even expressing admiration for Russian President Vladimir Putin, while dismissing NATO as "obsolete.

Source: Ahram online