Isis did not shoot down a US-led coalition warplane before capturing its Jordanian pilot, Washington has stressed.
Earlier Isis militants claimed it had targeted the plane with a heat-seeking missile.
But the US military has hit back at the suggestion, by clarifying that this was not the case and the plane was instead lost in territory held the group, before reiterating that it is a "terrorist organisation".
"Evidence clearly indicates that ISIL [Isis] did not down the aircraft as the terrorist organisation is claiming," the US military's Central Command said in a statement.
The Britain-based Syrian Observatory for Human Rights reported earlier that the aircraft was shot down yesterday near the northern province of Raqqa, which is almost entirely under the control of Isis.
Relatives of the pilot, believed to be First Lieutenant Muath al-Kasaesbeh, 26, said he was captured by fighters after his plane was downed. Two relatives told Reuters they were notified by the head of the Jordanian air force of his capture.
The incident makes Al-Kasaesbeh the first foreign soldier to fall into the extremist's hands, and the first coalition aircraft to be lost in Isis territory since strikes started in September.
The Jordanian military said in a statement read on state television today that its air force was carrying out a military mission against Isis "hideouts" on Wednesday morning, when "one of our warplanes crashed."
"The pilot was taken hostage by the Daesh terrorist organisation," it added, using the Arabic acronym for the Islamic State group.
It said IS and "those who support it" will be responsible for the safety of the pilot.
Later, Jordan's Information Minister Mohammad Al-Momani told satellite TV station Al Hadath the jet fighter "was shot at from the ground by rocket missiles and was brought down" and that an attempt to rescue the pilot before he was captured failed.
Al-Momani vowed that "the war on terrorism will continue," saying, the fight with the extremists was "to defend the Islamic religion."
The statement went on to describe Isis as a group that "does not want to conceal its terrorist plots" and which commits "many criminal acts from wanton destruction to killing innocent Muslim and non-Muslims in Syria and Iraq".
The pro-Isis Raqqa Media Centre (RMC) said the plane was downed near the village of Hamra Ghannam, outside Raqqa.
The organisation posted pictures of the pilot surrounded by more than a dozen militants, some masked, shortly after he was captured. It also released images of the militants posing with pieces of wreckage.
Another image published by the RMC showed the man — wearing only a white shirt and soaking wet — being captured by three gunmen as he was taken out of what appeared to be a lake. Al-Kasaesbeh's relatives verified these images.
A friend said Al-Kasaesbeh, who is from a prominent Jordanian family, was strongly committed to his mission and felt it was his religious duty to fight extremist groups that are "distorting the true spirit of Islam".
AL RAI Chief International Correspondent shared a picture of what he said was the captured pilot meeting the King of Jordan.
RMC later posted a photograph of the Jordanian military identity card of the pilot identifying him as Mu'ath Safi Yousef al-Kaseasbeh who was born on May 29, 1988.
The Raqqa Media Center said Isis fighters are searching the area in case there is a second pilot.
The previous claims by Isis were at first thought to be credible, as activists believe Isis have Igla missile systems, which were either taken or bought from rival Syrian rebels, or looted fro Syrian or Iraqi state arsenals.
The US-led coalition, which includes has carried out hundreds of air strikes against Isis positions in Syria since 23 September,
Saudi Arabia, Jordan, Bahrain and the United Arab Emirates have joined in air strikes against the extremist group, while Qatar is providing logistical support.
A staunch US ally, Jordan has also acted as a logistics base for the US-led campaign, and is a hub for intelligence-gathering operations against the jihadists, a western diplomatic source told Reuters.
Earlier, a Pentagon official said any further questions about the alleged capture should be referred to the government of Jordan. He spoke on the condition of anonymity because he was not authorized to discuss the reports by name.
Additional reporting by AP and Reuters
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