A Jordanian plane that crashed in Syria was not shot down by Islamic State militants, the US has said.

The pilot of the downed plane was identified as 26-year-old Jordanian Lieutenant Muath Safi Al-Kaseasbeh (Maaz Yusuf Al-Kasasbeh).

The US claims that the Jordanian pilot was captured by Islamic State militants after the plane was downed.

A Washington official said the allies were investigating whether the plane was shot down or suffered a mechanical fault.

Isis supporters, meanwhile are claiming the plane was hit by a heat-seeking missile near Raqqa city, an IS stronghold in northern Syria,

Jordan initially indicated the plane was shot down during a "military mission against the hideouts of the terrorist group" but later said it was unclear how the plane came to crash.

An army statement read on Jordanian state television said: "Jordan holds the group and its supporters responsible for the safety of the pilot and his life."

UN Secretary-General Ban Ki-moon has urged IS captors to treat the pilot "in accordance with international humanitarian law."

IS in Raqqa published photographs on jihadist websites purporting to show its fighters holding the pilot captive. An image of his military card was also published identifying him.

His father, Yusuf al-Kasasbeh, said the family had been informed of Lt Al-Kasasbeh's capture by the head of the Jordanian air force.

He appealed to Isis' leaders for compassion. "May Allah plant mercy in your hearts and may you release my son," he said.

British Foreign Secretary Philip Hammond wrote on Twitter that he was "concerned" for the pilot.