President Donald Trump said Thursday that the Iranian downing of a US drone over the Strait of Hormuz was likely the result of a "stupid mistake," though he left the door open to possible reprisals against Tehran.

"I find it hard to believe it was intentional if you want to know the truth," he told reporters while welcoming Canadian Prime Minister Justin Trudeau to the Oval Office.

"It could have been somebody who was loose and stupid that did it," Trump said.

When asked whether Washington planned to retaliate, the president said: "You'll soon find out."

In his initial public reaction, posted earlier Thursday on Twitter, the president appeared to treat the downing of the drone as a development of strategic significance.

"Iran made a very big mistake," he wrote.

But the president's view seemed to have softened by the time of the Oval Office encounter with journalists.

"I have a feeling - and I may be wrong and I may be right but I'm right a lot - that it was a mistake made by somebody that shouldn't have been doing what they do," Trump said. "I think they made a mistake and I'm not just talking about the country made a mistake somebody under the command of the country made a mistake."

He added, however, that the United States "will not stand for it."

Regarding policy toward Iran, Trump denied media accounts that some of his advisers were pushing him toward war with Tehran, though he did recall that he campaigned in 2016 on a promise to do away with "endless wars" such as the conflict in Afghanistan, now in its 18th year.

The US Navy denied Thursday that the surveillance drone shot down by Iran the day before had entered Iranian airspace.

Iran's Islamic Revolutionary Guard Corps said earlier Thursday it had brought down the US Navy's MQ-4C Triton drone after it entered the country's airspace and flew over the southern coastal province of Hormozgan.

"Iranian reports that the aircraft was over Iran are false," Navy Capt. Bill Urban, spokesman for the US Central Command, which oversees military operations in the Middle East, said in a statement. "This was an unprovoked attack on a US surveillance asset in international airspace."

The statement confirmed that the drone was shot down by an Iranian surface-to-air missile at around 11.35 GMT on Wednesday.

Tensions between Tehran and Washington have escalated in recent weeks, with the US deciding to send additional troops, ships and missiles to the Persian Gulf.

The US has accused Iran of a series of alleged sabotage attacks against oil tankers in the Strait of Hormuz, most recently on June 13 when the Japanese-operated Kokuka Courageous and Norwegian-owned Front Altair were hit by explosives.

Iran has denied any involvement in the incidents and has instead accused the US of trying to destabilize the region.

Few governments have joined Washington in blaming Iran for the tanker attacks and the owners of the Kokuka Courageous contradicted the Pentagon's account of the strike on that vessel.

Iran recently announced that it would breach a central pillar of the 2015 international nuclear deal by producing more than its 300-kilogram limit of enriched uranium.

In May 2018, Trump withdrew from the deal, which saw Iran scale down its nuclear-power program in return for sanctions relief.

His administration then went on to re-apply sanctions on Tehran, focusing on the Islamic Republic's banking and oil sectors, but also clamping down on Iran's ability to export its excess enriched uranium.

The US move to block uranium exports left Tehran with a choice of either ending enrichment entirely or exceeding the stockpile limit.

In response, Iranian President Hassan Rouhani said he would wind down his own observance of the pact, and gave the remaining European signatories, the United Kingdom, Germany and France, 60 days to come up with a mechanism that would allow Tehran to circumvent US sanctions.