Sen. John McCain made an unannounced trip to northern Syria last week to meet with U.S. forces stationed there, his office announced on Wednesday.
McCain, the chairman of the Senate Armed Services Committee, went to discuss the campaign for defeating militants from the Islamic State.
"Sen. McCain's visit was a valuable opportunity to assess dynamic conditions on the ground in Syria and Iraq," according to a statement from spokeswoman Julie Tarallo.
The trip, which is considered official travel, was first reported by The Wall Street Journal.
McCain, R-Ariz., has been one of President Trump's harshest critics.
The Associated Press reports that McCain recently declared Trump's administration in disarray and expressed concern over how national security decisions are being handled.
"During a speech Friday at the Munich Security Conference, McCain delivered a withering critique of Trump's worldview as he lamented a shift in the U.S. and Europe away from the 'universal values' that forged the Western alliance 70 years ago.
"McCain on Monday welcomed Trump's selection of Army Lt. Gen. H.R. McMaster to be his national security adviser, calling the pick an "outstanding choice."
The trip to Syria came as a battle ramps up to oust ISIS militants from Raqqa, the capital of the Islamic State's self-declared caliphate.
The statement from McCain's office went on to read, "The president has rightly ordered a review of U.S. strategy and plans to defeat" the Islamic State group and McCain looks forward to working with the administration and military leaders "to optimize our approach."
The U.S. Department of Defense is expected to soon send the White House a strategy to defeat ISIS.
In 2013, McCain made another trip to Syria, where he met with rebels trying to oust Syrian President Bashar Assad's forces.
Last month, Rep. Tulsi Gabbard, D-Hawaii, traveled to Syria and met with Assad while she was there.
The U.S. doesn't have diplomatic relations with Syria, and Gabbard's trip generated a lot of backlash.
She told NPR's David Greene in an interview on Morning Edition on Feb. 6:
"I felt it was important to take the meeting simply because I think we've got to be ready to meet with anyone if there is a chance that it can help bring about an end to this war, which is causing the Syrian people so much suffering. This is really what's at stake here, whether you think Assad is a good guy or a bad guy. Whatever you think about him, the fact remains that if we profess to care about the Syrian people's suffering — we've professed to care about peace — we've got to be able to do whatever it takes in order to achieve that."
Critics say Gabbard gave Assad credibility by meeting with him.