Fighters of the Islamic State (Isis) in Syria have scored a major propaganda coup by capturing a Jordanian air force pilot whose plane came down on Wednesday during an air raid by the international coalition near the northern city of Raqqa, the de facto jihadi capital.

Images posted on social media showed jubilant Isis gunmen, some of them masked, with a clearly frightened man, naked from the waist down and being dragged out of a lake. He was identified as the downed pilot and named on Twitter, which displayed his military ID card, as First Lieutenant Muadh al-Kasasbeh, 26. The Jordanian military immediately described him as a “hostage”.

The F-16 was the first warplane lost since the US-led coalition began air strikes against Isis in Syria three months ago. Both the jihadists and activists reporting to the Britain-based Syrian Observatory for Human Rights said the plane had been hit by an anti-aircraft missile.

But the US military dismissed the claim, saying “evidence clearly suggests that Isil [Islamic State] did not down the aircraft”. Another image on social media showing the plane’s intact cockpit canopy suggested that the pilot might have ejected.

Hundreds of coalition air attacks have helped stem Isis advances – though more successfully in Iraq than in Syria, where they have been criticised for weakening more moderate rebel groups fighting President Bashar al-Assad.

The Jordanian military issued a statement confirming the capture by Isis and saying it “holds the group and its supporters responsible for the safety of the pilot and his life”. It did not name him. “During a mission on Wednesday morning conducted by several Jordanian air force planes against hideouts of the IS terrorist organisation in the Raqqa region, one of the planes went down and the pilot was taken hostage,”, the Petra news agency quoted a source from the military’s general staff as saying. The Jordanian government went into emergency session to discuss its response.

The pilot’s father, Yousef al-Kasasbeh, appealed to Isis in an interview with a Jordanian website, Saraya, saying: “May Allah plant mercy in your hearts and may you release my son.” He also urged King Abdullah to bring him home.

Jordan is one of four Arab countries – the others are Saudi Arabia, Bahrain and the United Arab Emirates – which have been taking part in coalition attacks. Qatar is providing logistical support. But Jordan is in an especially vulnerable position: it is the only one of what the US calls the Arab “partner nations” which borders on both Syria and Iraq. It has taken in hundreds of thousands of Syrian refugees and there is sympathy and even support among Jordanian extremists for what is seen as an Isis fightback against Assad, Iran and Shia sectarianism.

An estimated 2,000-2,500 Jordanians are known to be fighting with Isis – the third largest foreign Arab contingent after Saudi Arabia and Tunisia.

King Abdullah has been an enthusiastic participant in the coalition, describing an elemental struggle between Muslim moderation and jihadi extremism. Jordan’s much-vaunted intelligence service is thought to be playing an important clandestine role in the anti-Isis campaign. But the Jordanian government has not advertised its military involvement, perhaps fearing revenge attacks by Isis or a domestic backlash. The capture of the pilot and his obvious propaganda value to the jihadis may well now highlight the risks involved.

Australia, Belgium, Canada, Denmark, France, the Netherlands and UK have joined the US in conducting air strikes on Isis in Iraq. The US and its four Arab allies, flying sorties in Syria, will all be concerned about any new Isis capability to bring down their planes.