The statement attributed to the "Islamic State" (IS) group appeared on a website the militant group commonly uses. It said an American female hostage was killed in Jordanian airstrikes near the northern Syrian city of Raqqa, the group's main stronghold.

The militants said the woman was buried beneath rubble after the raid, and published pictures of the alleged bombed site.

"The plane from the crusader coalition bombed a position outside the city of Raqqa after Friday prayers," the statement said.

Officials in the US made a brief comment, but did not elaborate.

"I cannot confirm those reports in any way. I'm not going into specifics of Americans held overseas," State Department deputy spokeswoman Marie Harf said.

The report comes a day after Jordan bombed IS targets in retaliation for the group's killing of a young Jordanian pilot, Muath al-Kaseasbeh, who was burned alive. Amman had promised a swift and lethal response to the video showing the man's murder.

The strikes were confirmed by the Jordanian military, but where they took place was not disclosed.

Jordan is part of a US-led coalition of around 60 countries that have bombed IS targets in Iraq and Syria, where the group controls large areas. But until now, Jordanian warplanes are only known to have carried out strikes in Syria.

Thousands of Jordanians turned out on the streets of Amman on Friday to denounce the Jordanian pilot's execution.

jr/kms (Reuters, AFP)