The beheaded US journalist Steven Sotloff fasted on Yom Kippur, the Jewish Day of Atonement, and prayed in the direction of Jerusalem while he was held captive by the Islamic State militants, says a fellow hostage.

Though he ran the risk of being discovered by the Islamists, Sotloff went ahead with his Jewish practices but kept them secret.

According to a report on Israel's Yediot Aharonoth, the witness said: "He [Sotloff] told them he was ill and didn't want to eat, even though they brought us eggs that day."

"It looked like he was praying in a hidden way towards Jerusalem. He noted what way the Muslims were praying in and changed his direction slightly."

Sotloff, who attended a Jewish school in Miami, was beheaded by a jihadist fighter sparking condemnation from across the world.

He was taken hostage by the Islamists on 4 August last year in Aleppo while covering the Syrian civil war.

The UN Secretary General Ban Ki-moon has said the world is "outraged" by the execution of the American journalist.

"We are all outraged at reports from Iraq about the brutal killing of civilians by Isis (IS), including yesterday's reported brutal beheading of another journalist. I strongly condemn all such despicable crimes and I refuse to accept that whole communities can be threatened by atrocity crimes because of who they are or what they believe," the UN chief said during a public address in New Zealand.