Kyiv, Ukraine - Ukraine's top security official said on Thursday that its passenger airliner that crashed in Iran, killing all 176 passengers and crew on board, may have been downed by a Russian-made missile.

The plane, carrying mostly Iranians and Iranian-Canadians, crashed on Wednesday amid a spiralling escalation between Tehran and Washington.

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A team of 45 Ukrainian experts has arrived in the Iranian capital to examine why the Ukraine International Airlines plane plunged from the sky minutes after takeoff from Imam Khomeini International Airport, Oleksiy Danilov, secretary of Ukraine's National Security and Defense Council, told journalists.

Meanwhile, US officials have claimed that the plane was most likely brought down accidentally by Iranian air defences, as President Donald Trump said he had a "terrible feeling" about the tragedy while offering no evidence.

"Somebody could have made a mistake," Trump told reporters, adding that he had suspicions about the crash but giving no other details.

Iran's head of civil aviation, Ali Abedzadeh, has dismissed these suggestions as "illogical rumours", according to Iran's ISNA news agency.

Ukraine floats theories

Danilov, the Ukraine security official, said one of the considerations was whether "the plane was hit by an air defence missile, including a [Russian-made] Tor cruise missile."

Moscow has for decades supplied Tehran with sophisticated weaponry, and Iran tested the Tor-M1 missiles in 2007.

Other possible reasons include "an explosion inside the plane as a result of a terrorist attack", a collision with another flying object or the explosion of one of the engines, he said.

The Ukrainian team "intends to look for the debris of a Russian Tor air defence missile," Danilov said.

Moscow-based analyst Alexey Malashenko, a respected Russian expert on the Middle East, rejected the allegation.

"I don't believe in this version, considering the craziness that is going on down there, it would have been version number one from the very start," he told Al Jazeera.

Ukrainian experts are cautious about the possible political bias of the ongoing investigation into the crash, which happened just hours after Iran launched missiles at US military facilities in neighbouring Iraq.

"Unfortunately, any official version will be subjected to doubt because of the specifics of the investigation," Kyiv-based analyst Alexey Kushch told Al Jazeera.

The disaster "is not a topic for hype on social media, sensationalism or conspiracy theories", Ukraine's President Volodymyr Zelenskyy said in a national address.

"We must investigate all possible causes," he wrote on Facebook on Wednesday, shortly after Iranian officials said the crash was caused by a fire that struck one of the plane's engines, causing the pilots to lose control.

Shortly before Zelenskyy's announcement, Ukraine's embassy in Iran removed a statement that ruled out the attacks from its website, Ukrainian media reported.

The Boeing 737-800 was last seen on radar at 2,400 metres, according to the Flightradar24 monitoring website, hours after Tehran launched more than a dozen ballistic missiles at US facilities in neighbouring Iraq.

Earlier, Iran said the plane caught fire and was trying to turn back, but its crew did not make a call for help.