WASHINGTON (Reuters) - U.S. Republican Senator Marco Rubio reluctantly backed President Donald Trump's nominee for secretary of state, Rex Tillerson, in a move that all but secures Senate confirmation of the former Exxon Mobil Corp chief executive as the nation's top diplomat later on Monday.

Rubio said he was troubled by Tillerson's recent responses before lawmakers regarding Russia as well as other countries, but that he ultimately decided he would vote to approve the nominee in deference to Trump, as well as to fill a critical top job.

"Despite my reservations, I will support Mr. Tillerson’s nomination in committee and in the full Senate," Rubio said in a statement ahead of the Senate Foreign Relations Committee's vote on Tillerson.

A vote in the Republican-controlled Senate was expected shortly after the committee vote. Tillerson was expected to win confirmation, despite opposition from Democrats.

Rubio, a onetime rival to Trump for the Republican presidential nomination, said he would not be so deferential regarding Trump's other nominees for top State Department posts.

At a confirmation hearing earlier this month, Tillerson's views appeared to be at odds with Trump's on key foreign policy issues such as nuclear proliferation, trade deals, climate change and relations with Mexico.

The nominee also had a tense exchange with Rubio on whether he believed Russian President Vladimir Putin was a war criminal, in reference to Russia's military actions in support of Syria's government. Tillerson said he would not use that term.

Tillerson, 64, also sidestepped questions on human rights, declining to condemn countries like Saudi Arabia and the Philippines for rights abuses.

Over the weekend, fellow Republican Senators John McCain and Lindsey Graham said they would back Tillerson despite their concerns over his relationship with Putin as CEO of one of the world's largest corporations.

Republican Senator Rob Portman on Monday also voiced his support and said he was reassured by Tillerson's commitment at the Jan. 11 hearing to support NATO, an organization that Trump has voiced differing opinions about.

Earlier on Monday, the most senior Democrat on the Senate Foreign Relations Committee, Ben Cardin, said he would not support Tillerson.

"The power of the Secretary of State to call out wrong, to name and shame, and to fight each day on behalf of the American people and freedom-seeking people the world over is an enduring symbol," Cardin said in a prepared statement.

(Reporting by Patricia Zengerle; Additional reporting by Susan Heavey; Editing by Peter Cooney and Jonathan Oatis)