US President Barack Obama has condemned the beheading of Steven Sotloff, the second American journalist killed by the Islamic State group.
Obama issued the statement on Wednesday, after the US National Security Council confirmed the authenticity of an online video showing the killing of Sotloff.
"Whatever these murderers think they will achieve by killing innocent Americans like Steven, they have already failed," Obama said.
"We will not be intimidated. Their horrific acts only unite us as a country, and, stiffened our resolve to fight against these terrorists."
The news comes as the Israeli government confirmed on Wednesday that Sotloff also held a Israeli citizenship, adding that it withheld the information in a bid to stem the risks to the captive.
"Cleared for publication: Steven Sotloff was #Israel citizen RIP," tweeted Paul Hirschson, a spokesman for the foreign ministry in Jerusalem.
Israeli media reports said the US-born Sotloff emigrated to Israel in 2005 and studied at the Interdisciplinary Centre
Herzliya, a private college near Tel Aviv.
On Tuesday evening, the Islamic State group released the video of what it said was the 31-year-old's death.
Sotloff had been held hostage in Syria by the group since August 2013.
British Prime Minister David Cameron has condemned the killing as an "absolutely disgusting, despicable act".
Sotloff's death comes two weeks after the group released a video showing the murder of James Foley, another US journalist, which a masked member of the Islamic State claims was revenge for US attacks on its fighters in Iraq.
In that video, Sotloff was pictured, with the fighter saying his life depended on Obama's next move.
US troops to Baghdad
The killer in it appears to be the same man in both videos - a British-accented man dressed in black fatigues and balaclava.
Internet monitors who have viewed both videos said that Sotloff's hair was longer in the second.
Less than a week ago, Sotloff's mother, Shirley, directly addressed the group's leader Abu Bakr al-Baghdadi in an appeal for his release.
"You, the caliph, can grant amnesty. I ask you, please, release my child. I ask you to use your authority to spare his life," Sotloff's mother said in the video.
On August 24, al-Qaeda's Syrian affiliate the al-Nusra Front freed the US writer, Peter Theo Curtis, who had been missing since 2012, following what officials said were efforts by the Gulf state of Qatar. He has since returned to the US.
Meanwhile, Obama has approved sending 350 more troops to help protect the US embassy in Baghdad and its support facilities in the Iraqi capital, raising the number of US forces to more than 1,000, the White House said in a statement.
The additional troops will not serve in a combat role, the White House said. Most are from the army and some are Marines, the Pentagon said on Wednesday.
The additional troops will include a headquarters element, medical personnel, associated helicopters and an air liaison team, said Rear Admiral John Kirby, the Pentagon's spokesman.