WASHINGTON/TEHRAN — US President Donald Trump said Thursday he had "suspicions" about the crash of a Ukrainian airliner outside Tehran as US media reported it had been mistakenly shot down by Iran, but Tehran quickly responded to this by ruling out a missile strike as the cause of a Ukrainian passenger plane crash, saying such a scenario made "no sense".

Trump said Thursday the United States had imposed new sanctions on Iran following missile strikes on bases housing US troops in Iraq that resulted in no American or Iraqi deaths.

"It's already been done. We've increased them. They were very severe, but now it's increased substantially," Trump said, without offering any specifics.

Trump had promised the "additional punishing sanctions" in an address to the nation Wednesday in retaliation for the attack — seen by experts as a measured first response by Tehran to the killing of Iran's top Gen. Qasem Soleimani in an American drone strike in Baghdad.

Meanwhile, unnamed officials told American media that Iranian air defense systems likely accidentally shot down the airliner Wednesday, killing all of the 176 people on board.

Newsweek, CBS and CNN said that satellite, radar and electronic data indicated the tragic error, which followed a ballistic missile barrage by Iran on two military bases in Iraq where US troops work.

Trump didn't directly confirm that conclusion, but strongly hinted at it.

"I have my suspicions," Trump said. "It was flying in a pretty rough neighborhood and somebody could have made a mistake."

"Some people say it was mechanical. I personally don't think that's even a question," Trump said, adding that "something very terrible happened."

Analysts pointed to pictures shared widely online of the wrecked fuselage of the aircraft showing multiple apparent puncture holes consistent with a rocket that detonated just outside the plane, blasting shrapnel into it.

"Similar marks were visible on wreckage of MH17," CNN reporter Jim Sciutto said, referring to the Malaysian Airlines flight which was shot down only July 17, 2014 over Eastern Ukraine by a Russian-designed surface-to-air missile.

But, Iran on Thursday ruled out the missile strike, saying such a scenario made "no sense".

The plane crashed shortly after take off Wednesday, killing all 176 people on board, shortly after Iran fired a volley of missiles against military bases in Iraq housing US personnel.

"Several internal and international flights were flying at the same time in Iranian airspace at the same altitude of 8,000 feet (2,440 meters)," Iran's Transport Ministry said.

"This story of a missile striking a plane cannot be correct at all," it said in a statement.

"Such rumors make no sense," Ali Abedzadeh, head of Iran's civil aviation organization and deputy transport minister, said in the statement.

Abedzadeh was reacting to rumors on social networks that the Boeing 737 was hit by a missile fired by Iran's Revolutionary Guards.

He said Iran and Ukraine were in the process of "downloading information" from black boxes retrieved from the crash site.

"But if more specialized work is required to extract and analyze the data, we can do it in France or another country," he added.

On Wednesday, Iran's Mehr news agency — close to ultraconservatives — quoted Abedzadeh as saying Iran "would not give the black boxes to the Americans".

But the minister's statement on Thursday rejected "rumors of Iran's resistance to delivering the black boxes... to the US".

Iran is not obliged to have the black boxes analyzed in the US, but America is one of only a few countries — including France and Germany — capable of carrying out such work.

Iranian authorities say initial indications showed the plane had turned back after suffering a problem.

A team of Ukrainian experts flew in and joined the investigation on the ground Thursday.

Kiev said it was studying several scenarios, including an in-flight collision, a rocket strike, an engine explosion caused by a technical problem, and an onboard blast. — AFP