Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

America's smaller European allies have expressed concern about President Donald Trump's mixed signals on the U.S. commitment to protect them from Russia.

The uncertainty threatened to deepen this week when U.S. officials said Secretary of State Rex Tillerson planned to skip what would have been his first official meeting with NATO in April. On Tuesday, the State Department said Tillerson has a scheduling conflict and suggested alternative dates that morning.

Moving the date would require a 28-nation consensus. A NATO official told NBC News "we are in contact with the State Department on scheduling."

A State Department spokesman confirmed to NBC News that Tillerson will travel later in the month to a series of unspecified meetings in Russia.

Tillerson's trip to Moscow was not confirmed by the Russian side. Foreign Ministry spokeswoman Maria Zakharova wrote on her official Facebook page that "we will not be confirming or denying this information at this stage."

The last time a secretary of state did not attend a NATO foreign ministerial meeting was in 2003.

But acting State Department spokesman Mark Toner said Tillerson would meet with all of the alliance's members Wednesday when the Global Coalition to Counter ISIS gathers in Washington. NATO Secretary General Jens Stoltenberg will also attend.

"While [Tillerson] won't specifically be discussing in the group session all of NATO’s equities, obviously he will have the opportunity to do pull asides with many of these countries," Toner said.

Trump, himself, meanwhile, still intends to travel to Brussels, Belgium, in May for a meeting of NATO heads of state, the White House said Tuesday night. It confirmed May 25 as the date for the visit, which was first announced in February.

Related: NATO Critic Trump Agrees to Attend Brussels Summit in May

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Tillerson's decision will likely raise eyebrows among some of the United States' European partners.

Close to the Russian border, some people fear that Trump's thawing relationship with President Vladimir Putin could leave them exposed. Like NATO, Moscow has been ramping its military exercises and also annexed Crimea from Ukraine in 2014.

Related: Trump's NATO Stance Has Town Miles From Russia 'Ready for War'

"Donald Trump's administration is making a grave error that will shake the confidence of America's most important alliance and feed the concern that this administration simply too cozy with Vladimir Putin," said U.S. Rep. Eliot L. Engel, D- New York, who was one of the first to respond to Tillerson's decision.

He added: "I cannot fathom why the administration would pursue this course except to signal a change in American foreign policy that draws our country away from western democracy's most important institutions and aligns the United States more closely with the autocratic regime in the Kremlin."

He also labeled the move "an absolute disgrace."

Historically, NATO's promise to come to the defense of any partner under attack has reassured these countries, particularly those in the former Soviet Union, who believed they would be safe from Kremlin interference.

That's shifted under Trump, who has spoken warmly of Putin, called NATO "obsolete" and suggested he would not protect allies unless they upped their military spending. The FBI is also investigating links between his election campaign and Russian hacking. Tillerson has had dealings with Putin in his former role as ExxonMobil CEO.

Secretary of State Rex Tillerson (C) Kevin Lamarque / Reuters / Reuters

Trump has recently spoken in more reassuring terms toward NATO, but many analysts say that even the suggestion that the U.S. might not respond to an aggression might leave allies vulnerable. Secretary of Defense James Mattis, at a brief photo spray Tuesday with his Finnish counterpart, expressed confidence that the U.S. will be represented at the upcoming NATO meeting.

"We'll take care of the representation," Mattis said. "This is something to be worked out, no problem."

2 hours of excellent talks with Sec.Mattis. Serious business but a very warm welcome. Always glad to be with such a strong friend of NATO. pic.twitter.com/gnnGz99UHh — Jens Stoltenberg (@jensstoltenberg) March 21, 2017

It appeared that other U.S. officials were well aware of the negative signals likely generated by Tillerson’s absence from the key meeting. Before the spokesman's official comment, one State Department told NBC News on condition of anonymity that there was a push within the agency to convince him to attend.

However, Tillerson is instead sending the State Department's second-most senior official, Tom Shannon, to the key NATO meeting on April 5-6.

A NATO official pointed out that "all allies are represented at NATO ministerial meetings ... it’s up to allies to decide at what level they are represented," adding that such gatherings were "important regular events."

Reuters, the news agency that first reported Tillerson's decision, quoted unidentified officials saying that the secretary of state would be staying in the U.S. to attending meetings between Trump and Chinese President Xi Jinping at the president's Mar-a-Lago resort in Florida.

Related: Chinese Media Claims Tillerson Visit Was a Home Run — for China

The State Department spokesman did not mention this detail and NBC News could not immediately independently confirm it.

The secretary of state is also scheduled to travel to a meeting of the G-7 countries in Italy before traveling to Russia next month.

While she did not confirm Tillerson's meetings in Russia, Zakharova, the Russian Foreign Ministry spokeswoman, said that "we are definitely surprised by the constant leaks of sensitive information from Washington."