Obama vows US will not be intimidated by IS' beheading of Steven Sotloff

President Barack Obama vowed on Wednesday that the US would not be intimidated by the beheading of a second American reporter but acknowledged the fight against the Islamic State (IS) would take time.

Obama pledged that justice will be served against the IS killers of 31-year-old reporter Steven Sotloff, wherever they hid and however long it took.

But he warned that eliminating the threat posed to the region by the group, from its bases in Iraq and Syria, would take time.

IS posted video footage online of Sotloff's beheading, confirmed as authentic by Washington, which sparked outrage around the world.

It said the journalist's killing, which comes on the heels of the beheading last month of another US reporter, James Foley, was in retaliation for expanded US air strikes against its fighters in Iraq over the past week.

It warned a British hostage would be next unless London backs off from its support for Washington's air campaign.

Prime Minister David Cameron said that Britain too refused to be cowed, while Foreign Minister Philip Hammond said that British air strikes were not being ruled out.

Obama said the whole world had been repulsed by the nature of Sotloff's murder but "we will not be intimidated".

"Those who make the mistake of harming Americans will learn that we will not forget and that our reach is long and that justice will be served," he said.

Obama said Washington was determined to halt the IS threat but warned it would depend on close cooperation with partners in the region.

He has previously admitted that his administration has yet to develop a comprehensive strategy for tackling IS on both sides of the Iraq-Syria border.

Washington has ruled out any air strikes for now against IS fighters on the Syrian side, where they hold a swathe of the east.

Obama has also ruled out any cooperation with the Damascus regime against IS, for fear that it would drive other Sunni rebel groups in Syria into alliance with the jihadists.

'Despicable act'

The British prime minister said the beheading video depicted an "absolutely disgusting, despicable act" and chaired a meeting of security chiefs to discuss how to tackle the IS threat.

The masked executioner in the video spoke with a London accent and claimed to be the same man, confirmed by UK security services as a Briton, who beheaded Foley.

"I'm back, Obama, and I'm back because of your arrogant foreign policy towards the Islamic State," the black-clad militant says, wielding a combat knife.

"So just as your missiles continue to strike the necks of our people, our knife will continue to strike the necks of your people," he declares, before reaching round to cut his captive's throat.

At the end of the three-minute recording, the militant threatens another captive, identified as Briton David Cawthorne Haines.

"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," he says.

Britain has maintained a media silence about the kidnapping of aid worker Cawthorne Haines and there were few immediate details about when or how he was abducted.

"I can assure you that we will look at every possible option to protect this person," the British foreign minister said.

"We have to deal with [IS] on the basis of the wider threat that they pose to the British public as well as this individual," Hammond added.

"If we judge that air strikes could be beneficial, could be the best way to do that, then we will certainly consider them but we have made no decision to do so at the moment."

Britain has not so far carried out any strikes of its own against IS targets in Iraq but it has made extensive reconnaissance flights in support of the US air campaign from its base in Cyprus.

'Brave and talented'

In a statement, the Sotloff family, who live in Miami, said: "The family knows of this horrific tragedy and is grieving privately. There will be no public comment from the family during this difficult time."

Israeli media reported that the family was Jewish and that Sotloff himself held joint US-Israeli nationality but IS made no mention of either in its video.

Sotloff's former employers at Time and Foreign Policy paid tribute to a man widely respected for his intrepid reporting in Syria and the wider region, including a previous stint in Libya.

Hours after the posting of the video, the White House announced that Obama had authorised about 350 more US troops to beef up security at US diplomatic facilities and protect personnel in Baghdad.