The Trump administration ordered a partial withdrawal of its diplomats from Iraq on Wednesday, as Washington warned of heightened threats from Iran and Tehran-backed Shiite militias.

This week, a U.S. official said Iran likely was behind recent attacks on oil tankers in waters off the United Arab Emirates, something Tehran has strenuously denied. Last week, the administration deployed an aircraft carrier, a bomber task force and other equipment and personnel, citing unspecified threats from Iran.

But U.S. authorities haven’t disclosed details behind their assessments, and officials on Wednesday canceled an intelligence briefing scheduled for lawmakers on the House Intelligence Committee, prompting complaints on Capitol Hill.

“This threat stream, let me tell you, is real,” a senior State Department official told reporters, declining to provide details. “The Iranians are threatening American interests and interests of our allies, all throughout this region.”

The U.S. departure order, affecting an estimated hundreds of U.S. personnel in Baghdad and Erbil, was accompanied by more general State Department warnings to U.S. citizens in Lebanon and the U.A.E. People in those countries were advised to remain vigilant because of heightened tensions in the region.