A social media campaign of solidarity with the Jordanian pilot captured last week by Islamic State (Isis) has gone viral in Arab countries.

Muadh al-Kasasbeh ejected from his F-16 jet before it crashed near the de facto Isis ‘capital’ of Raqqa on Wednesday. Isis has published pictures of him naked from the waist down and with blood on his mouth as its fighters dragged him away from a body of water, believed to be the Euphrates river.

Within hours, messages of support for the 26-year-old first lieutenant began circulating on Twitter, Facebook and other social media sites with the hashtag, which is mainly in Arabic, #WeAreAllMuadh.

Jordan’s Queen Rania was one of the first to join, with an Instagram picture of a fighter jet formed from the letters of al-Kasasbeh’s first name. Others tweeted BringMuathHome, using an alternative spelling of his first name: “BringMuathHome he was fighting terrorists that hijacked our lands, our religion and slaughtered people,” said @omarkurdi20 on Twitter.

“I don’t support war or violence, but I do believe in defence against a violent group of psychopaths who’ve hijacked my religion,” said @rdawud, adding the Arabic #WeAreAllMuadh hashtag.

The campaign also drew in Isis sympathisers, however, who used the same hashtags to threaten the pilot and his supporters, taunting them that he would come back “in a million pieces”.

“If you have enough time to cry 4 a pilot who killed 100s of innocents, then understand that u lost not only islam but also humanity,” said @Muslims_Way, adding one of the hashtags.

Jordan has threatened Isis with “grave consequences” if anything happens to the young pilot, whose plane was the first to be brought down since the bombing campaign began.

Although there are several Arab nations in the US-led coalition – including the United Arab Emirates, Saudi Arabia, Bahrain and Qatar, as well as Jordan – most have played down their role in a campaign that some fear could prompt revenge attacks from Isis, and has angered more extreme sections of their own populations.

Both the Jordanian and US military have rejected Isis claims that the plane was shot down while flying low near the Euphrates, saying that it appeared to have suffered a technical fault.

“First indications show that the crash of the military plane in the area of the Syrian city of Raqqa was not caused by Daesh fire,” a Jordanian military official said, using another name for Isis.

“But since the wreckage of the plane cannot be reached and since its pilot is not present, we cannot at this moment determine the exact cause of the crash,” he said in remarks quoted by the AFP news agency.

Eyewitness reports from the area also suggested that a problem with the plane, rather than a missile attack, had forced the pilot to eject.

“[He] was flying at a high altitude to start with. He hit the brick factory and then disappeared from sight,” Obada al-Hussein, an activist in Raqqa, said. “Then the plane flew back, and this time smoke was coming out of it. I believe there was a technical failure.”

Al-Kasasbeh’s father has called on his captors to treat his son humanely and return him soon. “I direct a message to our generous brothers of the Islamic State in Syria: to host my son, the pilot Muadh, with generous hospitality,” he said. “I ask God that their hearts are gathered together with love, and that he is returned to his family, wife and mother … We are all Muslims.”

Isis have executed hundreds of prisoners of war captured in Syria and Iraq, most of them fellow Muslims, but may try to extract cash or a prisoner exchange in return for the pilot.