US President Donald Trump ordered an attack on Iran on Thursday in retaliation for the downing of a surveillance drone in the Strait of Hormuz, officials said, but he called the operation off just hours before it was due to occur.

Administration officials, who spoke on the condition of anonymity to discuss sensitive national security decisions, said the president approved the strikes after Iran's Islamic Revolutionary Guard Corps (IRGC) earlier in the day shot down a Navy RQ-4 drone operating off Iran's southern coast, a move Trump described as a "very big mistake."

But he later changed his mind, the officials said. It was not immediately clear why Trump decided to pull back the operation or what it would have included. The decision was first reported by The New York Times.

The aborted operation capped a day in which news of the drone's targeting heaped fuel on already heightened fears that the United States and Iran are on a course toward a military conflict as each side blamed the other for the incident.

Tehran and Washington gave conflicting accounts of what occurred when the massive drone crashed into the sea. While Iran said it had entered its airspace, the US military denied that assertion, characterizing the incident as an "unprovoked attack" over one of the world's most important commercial waterways.

In remarks alongside visiting Canadian Prime Minister Justin Trudeau at the White House, Trump condemned the shoot-down but also appeared to tamp down speculation that a counterstrike might be in the works, saying the drone may have been shot down without the knowledge of Iranian leaders.

"I'm not just talking about the country made a mistake. I'm talking about somebody under the command of that country made a mistake," Trump said at the White House. "I find it hard to believe it was intentional" on the part of Iran's top officials, the president said.

Trump was noncommittal about a US counterattack. "Let's see what happens," he said. "This is a new fly in the ointment - what happened, shooting down the drone - and this country will not stand for it."

The White House invited a bipartisan group of top congressional leaders to a meeting Thursday afternoon to discuss the situation. Among those invited were Senate Majority Leader Mitch McConnell, R-Ky, House Speaker Nancy Pelosi, D-Calif., Senate Minority Leader Charles Schumer, D-NY, House Minority Leader Kevin McCarthy, R-Calif, and the chairmen and ranking minority party members of the House and Senate intelligence and armed services committees.

"We had a good briefing," McConnell said, adding that he could confirm that an unmanned aerial vehicle "was fired on from Iranian soil and it was in international waters. And beyond that I think the administration is engaged in what I would call measured responses."

Schumer said he cautioned that "these conflicts have a way of escalating."