BAGHDAD- Islamic State jihadists have beheaded a second American reporter, releasing video of what it said was retaliation for expanded US air strikes in Iraq, hours before President Barack Obama ordered more troops to Baghdad.
Washington confirmed that the video of the execution of 31-year-old reporter Steven Sotloff, which a US spokeswoman described as "sickening", was authentic.
As Iraqi forces kept up their fightback against the jihadists northeast of Baghdad, IS warned that a British hostage would be next unless London backed off from support for Washington's air campaign in Iraq.
The masked executioner in the video spoke with a London accent, and claimed to be the same man, dubbed Jihadi John by the British press, who beheaded US journalist James Foley, 40, in a similar posting last month.
In the latest footage, Sotloff calmly addresses the camera to say he is a victim of Obama's decision to press on with air strikes against the jihadists in Iraq.
At the end of the five-minute video recording, discovered online by the SITE Intelligence monitoring group and seen by AFP, the militant threatens another captive, identified as Briton David Cawthorne Haines.
"I'm back, Obama, and I'm back because of your arrogant foreign policy towards the Islamic State," the black-clad jihadist says, wielding a combat knife.
The militant condemns US air strikes against IS fighters around both Mosul Dam in the north and the Shiite Turkmen town of Amerli further south, which dates the video to the past week.
"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.
In a warning to Britain, the killer declares: "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."
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.
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."
After Foley's death, Sotloff's mother Shirley had addressed a video message to IS leader Abu Bakr al-Baghdadi pleading for her son's life, and insisting he had no influence on US policy.
British Prime Minister David Cameron said the video depicted an "absolutely disgusting, despicable act" and convened a meeting of security chiefs to discuss how to tackle the IS threat.
'Brave and talented'
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.
Sharing on Twitter a piece Sotloff had once written from a besieged rebel enclave in the Syrian city of Aleppo, Foreign Policy's Middle East editor David Kenner dubbed him "a brave and talented reporter".
UN Secretary General Ban Ki-moon said the world was outraged at the apparent beheading.
"I strongly condemn all such despicable crimes and I refuse to accept that whole communities can be threatened by atrocities because of who they are or what they believe," Ban said.
Sotloff's murder comes on the heels of a brutal IS campaign of executions and abductions targeting Iraq's Christian and Yazidi minorities and increased pressure on Obama to toughen his stance against the group.
Obama has promised to be "relentless" in his protection of US citizens in Iraq but admitted last week that Washington does not yet have a strategy to deal with IS in its heartland in eastern Syria.
More US troops
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.
But Obama himself made no comment on Sotloff's murder or the situation in Iraq and Syria, as he arrived for talks in Estonia on Wednesday ahead of a NATO summit in Wales.
Pentagon Press Secretary Rear Admiral John Kirby said the approximately 350 additional military personnel on the ground in Baghdad would provide security at the embassy compound and its support facilities.
"This builds upon previous embassy security deployments announced on June 15 and June 30 and will bring the total forces responsible for augmenting diplomatic security in Iraq up to approximately 820," he said.
Washington initially limited the air support it launched on August 8 to Iraqi Kurdish forces fighting the jihadists in the north.
But late last week it expanded it to Iraqi troops and Shiite militia battling to relieve trapped civilians in Amerli, helping them to break the months-long IS siege on Sunday.
Assistance is now arriving in the town, brought in both by Shiite militia fighters and the United Nations, which said it had "delivered 45 metric tonnes of life-saving supplies".
But Washington has ruled out any air strikes for now against IS fighters in neighbouring Syria, where they hold a large 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.