Steven Sotloff, the kidnapped journalist beheaded by Islamic State, was a dual Israeli-US citizen who had managed to hide his second nationality from his militant Islamist captors even while reportedly managing to fast during the Yom Kippur religious festival.

Sotloff, 31, became the second kidnapped journalist to be beheaded by the group, which also murdered US journalist James Foley. Islamic State has threatened to kill a British hostage next.

The revelation of Sotloff's Israeli citizenship was disclosed by the country's foreign ministry on Wednesday morning. In a brief tweet, Paul Hirschson, a spokesman for the ministry, said: "Cleared for publication: Steven Sotloff was Israel citizen RIP."

It followed confirmation by the US that the video depicting his murder was genuine.

"The US intelligence community has analysed the recently released video showing US citizen Steven Sotloff and has reached the judgment that it is authentic," national security council spokeswoman Caitlin Hayden said in a statement.

The revelation that Sotloff also had Israeli nationality came as President Barack Obama said that the US would not be intimidated by Islamic State militants and would build a coalition to "degrade and destroy" the group.

Obama still did not give a timeline for deciding on a strategy to go after the extremist group's operations in Syria. "It'll take time to roll them back," he said at a news conference during a visit to Europe.

Sotloff appears to have been murdered by the same masked man with a British accent who was depicted killing Foley in a video made public by Islamic State on 19 August.

Sotloff's Israeli nationality had been not been revealed in the Israeli media during his captivity, while authorities had reportedly gone to extraordinary lengths to expunge any reference from the internet to his time spent in the country.

According to reports in the Israeli media, Sotloff had studied in Israel.

As fresh details emerged about Sotloff's life, colleagues paid tribute to his courage and dedication.

Sotloff grew up in Miami, the son of Arthur and Shirley Sotloff, and – according to the Jerusalem Post – had previously worked for Temple Beth Am day school in Florida.

Sotloff also had work published both by the Media Line and the Jerusalem Report, filing for the latter shortly before his kidnapping a year ago.

"We refused to acknowledge any relationship with him in case it was dangerous for him," explained Avi Hoffman, editor of the Jerusalem Report.

According to an interview in Yedioth Ahronoth with another former Islamic State captive, who befriended Sotloff in captivity, he successfully managed to hide the fact he was Jewish from his captors managing to fast for Yom Kippur by feigning sickness.

"He told them he was sick and doesn't want to eat, even though we were served eggs that day," the friend told the paper.

"He used to pray secretly in the direction of Jerusalem. He would see in which direction (his Muslim captors) were praying and then adjust the angle."

According to one report – in the Times of Israel – Sotloff, who had moved to Israel in 2008, later became somewhat disillusioned with his newly adopted country.

"Like most of us, he came here and he became very critical of the government," said Hillary Lynne Glaser, a former classmate who studied international relations at the Interdisciplinary Centre in Herzilya with Sotloff.

"I'm not so sure it was about the Israeli-Arab conflict, I think it was more how they treat their own people. But he still came back to visit. He didn't hate it enough to not come visit," she told the paper. "He still considered it his home."

The paper also quoted Oren Kessler, an Israeli journalist who had corresponded online with Sotloff during the Arab spring, saying that Sotloff had been careful not to reveal he was Jewish, opting instead to tell locals that he had been raised Muslim but secular, without a mosque affiliation, sometimes explaining that his name was Chechen.