UN Secretary General Ban Ki-moon on Wednesday expressed his "concern" over a Jordanian pilot captured by the Islamic State jihadist group after his plane went down in Syria.
The militants and a monitoring group both said the plane had been shot down with an anti-aircraft missile, but this has not been verified.
Ban condemned the pilot's kidnapping and called on the Islamic State group to release him.
"The secretary noted with concern the news of the downing of a Jordanian plane and of its pilot being taken prisoner," a statement from the United Nations said.
"He calls on his captors to treat the pilot in accordance with international humanitarian laws."
The IS group released photographs of the captured pilot and showed a military card identifying him as 26-year-old First Lieutenant Maaz al-Kassasbeh.
Jordan is a member of the US-led air campaign against the Islamic State jihadists, who have taken over huge swathes of territory in Syria and Iraq.
Ban urged "all sides to find a durable solution to the conflict."
It was the first coalition warplane lost since air strikes on the Islamic State jihadists began in Syria in September.
Coalition warplanes have carried out regular strikes around Raqa, which the IS organization has used as the headquarters for its self-declared "caliphate" after seizing control of large parts of Syria and Iraq.