Iran's paramilitary Islamic Revolutionary Guard Corps said Thursday it has shot down a U.S. drone in the southern province of Hormozgan facing the Persian Gulf, an act that drew the ire of U.S. President Donald Trump.

Speaking to reporters in Washington, Trump said Iran "made a big mistake," and that the unmanned drone was traveling "clearly over international waters."

The Global Hawk surveillance plane was shot down after it entered Iranian airspace, Iran's Press TV reported, citing a statement issued by the corps.

Asked how the United States would respond, Trump said, "You'll find out." He did not provide details.

The two sides differed as to the circumstances surrounding the incident. The IRGS said it shot down the drone over Iranian airspace, but the United States said it was flying in international airspace over the Strait of Hormuz in a freedom of navigation mission.

"This was an unprovoked attack on a U.S. surveillance asset that had not violated Iranian airspace at any time during its mission," said Lt. Gen. Joseph Guastella, commander of the U.S. Air Forces Central Command.

Guastella said Iranian reports that the aircraft was shot down over Iran "are categorically false."

Trump said he "would imagine it was a general or somebody that made a mistake in shooting that drone down."

He warned that if a craft carrying American troops had been targeted, "It would have made a big difference."

Tensions between the United States and Iran have heightened over last week's attacks on two oil tankers near the Strait of Hormuz, which Washington has blamed on Tehran. Iran has denied involvement.

The United States said Monday it was sending about 1,000 additional troops to the Middle East. It earlier sent an aircraft carrier group and B-52 strategic bombers to the region.

In New York on Thursday, U.N. spokesman Stephane Dujarric said Secretary General Antonio Guterres is "very concerned" and is calling on the parties to "exercise maximum restraint and avoid any action that could inflame the situation."

Dujarric said the secretary general reiterates "that the world cannot afford a major conflict in that area."