In a tweet on Friday, US President Donald Trump revealed a detailed aerial image of an Iranian missile launch pad, an unusual disclosure that may have confirmed the United States is violating Iran's airspace in order to spy on its missile programme.

Some imagery experts, examining the angle and very-high resolution of the image, said it may have been taken by an aircraft, possibly a drone.

"It looks like it was taken from an airborne platform, not a satellite," said Jeffrey Lewis, an arms control expert at the Middlebury Institute of International Studies at Monterey, an assessment echoed by several other experts.

It would come as no surprise to Iran that the United States is gathering images of missile sites and other facilities. But disputes over US violations of Iranian airspace have exacerbated tensions in the Persian Gulf.

In June, Iran shot down an unmanned US surveillance drone over the Strait of Hormuz that it said had flown into its airspace.

The United States of America was not involved in the catastrophic accident during final launch preparations for the Safir SLV Launch at Semnan Launch Site One in Iran. I wish Iran best wishes and good luck in determining what happened at Site One. pic.twitter.com/z0iDj2L0Y3

— Donald J. Trump (@realDonaldTrump) August 30, 2019

The United States said the plane was in international airspace. The shootdown prompted Trump to order a military strike on Iran, which he called off at nearly the last minute.

The image Trump tweeted on Friday is almost certainly highly classified, experts said, and bears markings that resemble those made by intelligence analysts. They note damage to the facility and vehicles near it, as well as "scorching and damage" on one side of the launch pad.

Trump said it showed a "catastrophic accident during final launch preparations for the Safir SLV Launch at Semnan Launch Site One in Iran." The Safir is an Iranian rocket used to place satellites in orbit.

Trump said that "The United States of America was not involved" in the incident, which was puzzling since Iran had one day earlier confirmed a rocket explosion at the site, which it said was "due to some technical issues."

"I wish Iran best wishes and good luck in determining what happened at Site One," Trump wrote, in a taunting jab.

What Trump shared on Twitter appears to show a camera flash and a person's shadow, leading to speculation that Trump or one of his aides may have snapped a picture of the image using a cellphone.

As it frequently does, the president's public schedule lists an intelligence briefing at 11.30 on Friday morning. Those sessions are typically done in the Oval Office when the president is in Washington. Trump's tweet had a time stamp of 1.44pm.

The White House declined to address questions about the tweet and the image, including whether it was classified, whether it was produced by an aircraft or drone and whether it had been displayed in a special secure room where hand-held electronics are forbidden.

The White House also declined to address why Trump chose to tweet the image and the direct message to Iran, and whether he himself took the photo.

Later, speaking to reporters on Friday evening, Trump said, "we had a photo and I released it, which I had the absolute right to do and we'll see what happens."

As president, Trump has the authority to declassify any information he wants. Usually, that decision is based on consultations with US intelligence agencies about the potential damage that disclosure could cause.

The United States rarely discloses intelligence-gathering operations or the details about them, because that can help an adversary understand how the United States is spying.

Trump frequently excoriates government officials who he says leak classified information to the press. Hours before posting the image of the Iranian launch pad, he sent several tweets ridiculing former FBI director James Comey for sharing memos about his interactions with Trump, some of which were determined to contain classified information.

The Justice Department inspector general found that Comey did not reveal any classified information publicly, but that one memo he shared with his attorneys was later determined to contain information that was classified as confidential, the lowest level of secrecy.

Trump has adopted shifting tones when discussing Iran in recent months, swinging from threats to an overt offer of diplomacy that Iran rejected in the wake of the drone incident.

Trump has said he could foresee negotiations with Iran's leadership to forge a replacement for the 2015 international agreement limiting Iran's nuclear programme. Trump says that deal does not go far enough and he could do better.

Both Trump and Iranian President Hassan Rouhani will be in New York in late September for the annual meeting of the United Nations General Assembly. Trump did not rule out a meeting there.

"I have a good feeling. I think he is going to want to meet and get their situation straightened out. They are hurting badly," Trump said.