By BEN HUBBARD and RANA F. SWEIS DEC. 25, 2014

BEIRUT, Lebanon — Jordan threatened the militants of the Islamic State on Thursday with “grave consequences” if they harmed a Jordanian pilot captured after his F-16 crashed in northern Syria.

The warning, issued by Jordan’s Parliament, came as members of the pilot’s family appealed to his captors to welcome him as a “guest” and to show him mercy as a fellow Muslim.

But no new information on the fate of the pilot, First Lt. Moaz al-Kasasbeh, has emerged since his jet went down on Wednesday and supporters of the Islamic State, also known as ISIS or ISIL, distributed photos online that showed him in his underwear and with a bloody mouth as bearded gunmen led him away.

His plane was the first to crash since an American-led coalition of countries, including the Arab nations of Jordan, Saudi Arabia, Qatar, Bahrain and the United Arab Emirates, began bombing Islamic State targets this year in an attempt to weaken the group’s hold of territory in Syria and Iraq.

Lieutenant Kasasbeh is also the first military member of the coalition to be captured by the militants, raising the prospect that the group could use him for propaganda purposes or kill him for revenge.

The Islamic State often distributes videos of its fighters executing captured Syrian and Iraqi soldiers and has beheaded two American and two British civilians in what it called revenge for their countries’ war against it.

The pilot’s capture has shocked Jordan, one of the United States’ closest Arab allies. Like all of the coalition’s member nations, Jordan has spoken generally about its participation in the campaign but has not elaborated on its role for fear of retribution by the Islamic State and to avoid provoking the jihadists’ domestic sympathizers.

It remains unclear whether the pilot’s aircraft had a mechanical failure or, as the Islamic State militants have claimed, was shot down with an antiaircraft missile. American military officials said Wednesday there was no indication a missile had felled the plane.

The Jordanian warning was issued by the lower house of Parliament, which said in a statement carried by the state-run Petra news agency that the Islamic State and its supporters would face “grave consequences if pilot First Lt. Moaz al-Kasasbeh is harmed.”

The statement voiced continued support for Jordan’s role in the coalition and urged the government of King Abdullah II to “do its utmost to ensure a safe return of the pilot.”

The king met with the pilot’s family on Wednesday, and his wife, Queen Rania, posted on Instagram an image of a Jordanian flag in the shape of a fighter jet with the hashtag “We are all Moaz.”

Images of the captive posted on the Internet by the group showed him appearing traumatized and disoriented as he was surrounded by triumphal militants. Some wore their customary face hoods, but others were clearly recognizable.

Lieutenant Kasasbeh’s father, Safi, said in an interview with Reuters Television on Thursday that he did not consider his son to be a hostage of the militants.

“I don’t call him a prisoner,” he said, asking the militants to treat his son well. “I call him a guest of our brothers in Syria, of the Islamic State group.”

The pilot’s uncle, Fahed al-Kasasbeh, a retired major general from Jordan’s armed forces, said by phone Thursday that the family had received no new information on Lieutenant Kasasbeh’s fate and appealed to the jihadists to welcome him as a fellow Muslim.

“We expect him to be treated the way the Prophet Muhammad treated his captives, with mercy and generosity,” he said.

That prospect was unclear at best. Supporters of the Islamic State have been taking to social media to suggest ways to kill Lieutenant Kasasbeh. The Raqqa Media Center, a pro-Islamic State group that distributed early photos of the captured pilot, posted a poll on its Facebook page asking what the militants should do with him. The options: negotiate for him, slaughter him with a knife, shoot him.

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