The fate of two hostages, Japanese journalist Kenji Goto and Jordanian air force pilot Mu'ath al-Kaseasbeh, remained unclear on Friday as a proposed swap between Jordan and the Islamic State group foundered on the suspicion that one of the captives might not in fact be alive.

On Thursday night, Jordanian government spokesman Mohammad al-Momani said Jordan would not proceed with the release of Sajida al-Rishawi, a failed suicide bomber jailed in Jordan, because the extremists had not provided proof that al-Kaseasbeh remains alive.

"We need a proof of life, so we cannot proceed with what we have announced yesterday regarding the exchange between Sajida and the Jordanian pilot," al-Momani told reporters late Thursday night.