WASHINGTON — The decision to send an aircraft carrier and a group of Air Force bombers to the Middle East was based in part on intelligence indications that Iran had moved short-range ballistic missiles by boat in waters off its shores, an American official said Tuesday.

The movement, first reported by CNN, was among a range of recent indications that Iran might be considering or preparing to attack U.S. forces in the region, said the official, who spoke on condition of anonymity in order to discuss sensitive intelligence.

The official said it was not clear whether the boats with missiles represented a new military capability that could be used against U.S. forces or were only being moved to shore locations.

When the White House announced Sunday that the USS Abraham Lincoln carrier strike group and a bomber task force were being deployed to the Middle East, John Bolton, the national security adviser to President Donald Trump, cited "troubling and escalatory indications and warnings" but did not explain what they were.

Bolton said the movement of additional military firepower to the Middle East was meant to send a "clear and unmistakable message to the Iranian regime that any attack on the United States interests or on those of our allies will be met with unrelenting force."

Patrick Shanahan, the acting secretary of defense, told reporters at the Pentagon on Tuesday that he had approved the expedited movement of the Lincoln strike group and the deployment of a bomber group based on "credible reporting" on Iran.

"What you see is us getting in the right posture for that dynamic environment" in Iraq and elsewhere in the region, Shanahan said. The U.S. has about 5,200 troops in Iraq.