After analysis by US intelligence, the White House determined on Wednesday that the video showing freelance journalist Steven Sotloff's beheading is authentic, according to National Security Council spokeswoman Caitlin Hayden.

Speaking in Estonia, US President Barack Obama strongly condemned Sotloff's brutal murder, saying that the Miami native had deeply loved the Islamic world and reported on the struggles of Muslims.

"Whatever these murders think they will achieve by killing innocent Americans like Steven, they have already failed," Obama said. "Their horrific acts only unite us and stiffen our resolve."

The 31-year-old Sotloff's work was published in "Time" and "Foreign Policy" magazines, and he had previously reported on the civil war in Libya. He disappeared in Syria in August of 2013.

Shirley Sotloff, Steven's mother, had published a video message last week asking the "Islamic State" to release her son.

Murderer with a British accent

On Tuesday, the "Islamic State" published the video of Steven Sotloff's murder, which was discovered online by the SITE terrorism monitoring group. A militant clad in black and speaking with a British accent threatened US President Barack Obama and America before beheading Sotloff.

"I'm back, Obama, and I'm back because of your arrogant foreign policy toward the Islamic State," the unidentified militant says, referring to the taped beheading of another US freelance journalist, 40-year-old James Foley.

"So just as your missiles continue to strike the necks of our people, our knife will continue to strike the necks of your people," the militant said.

In the video, Sotloff says that he is the victim of President Obama's decision to press on with airstrikes.

UK citizen threatened

Sotloff's murderer then threatened to kill British citizen David Cawthorne Haines, who reportedly worked for an aid group in Syria, if the US doesn't stop launching airstrikes against the Islamic State in northern Iraq.

"We take this opportunity to warn those governments that enter this evil alliance of America against the Islamic State to back off and leave our people alone," the black-clad militant says.

Meanwhile, British Prime Minister David Cameron convened an emergency meeting on Wednesday following Sotloff's beheading, condeming the journalist's murder as a "despicable and barbaric act."

"As I have said consistently over the last few weeks, ISIL terrorists speak for no religion," Cameron said, using an alternative name for the Islamic State. "They threaten Syrians, Iraqis, Americans and British people alike and make no distinction between Muslims, Christians or any other faith."

The militant who murdered Sotloff and Foley is thought to be a British national. Some 500 UK citizens have gone to Syria to fight with Islamist militants, according to intelligence and security services.

Cameron had previously announced new counterterrorism measures that would empower the government to seize the passports of Islamist fighters bound for Syria or Iraq.

slk/jr (AP, AFP, Reuters)