The so-called "Islamic State" has murdered a second American reporter, releasing another video Tuesday showing a masked militant with a British accent cutting the throat of a US captive.

In the latest footage, the 31-year-old reporter Steven Sotloff calmly addresses the camera to say he is a victim of President Barack Obama's decision to press on with air strikes against the jihadists.

At the end of the five-minute recording, discovered online by the SITE terrorism monitoring group and seen by AFP, the militant threatens another captive, identified on screen as a British citizen.

"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 and speaking in a London accent.

This was a reference to a video issued last month in which US journalist James Foley was murdered, again by a suspected British foreign fighter, and in almost identical fashion.

- 'Despicable act' -

The fighter condemned recent US air strikes on the area around the Mosul Dam in Iraq, dating the footage after 40-year-old Foley's killing.

"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 declared, before reaching round to cut his captive's throat.

Sotloff's hair and beard were longer than in the previous footage, in which he was threatened with death in retaliation for US strikes against IS forces.

In a warning to Britain, the killer declared: "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 Sotloff family, who live in Miami, issued a statement through a spokesman, Barak Barfi, that implicitly confirmed the video as authentic.

"The family knows of this horrific tragedy and is grieving privately. There will be no public comment from the family during this difficult time," it said.

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.

A US spokeswoman said Washington was trying to authenticate the "sickening" footage, and British Prime Minister David Cameron said it depicted an "absolutely disgusting, despicable act."

The latest brutal murder, which came on the heels of fresh reports of more IS atrocities against Iraqi and Syrian prisoners, will increase pressure on Obama to toughen his stance against the group.

He 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.

Obama left Washington on Tuesday for Europe, where he was due to meet NATO leaders later this week, and made no comment on Sotloff's murder or the situation in Iraq and Syria.

US air strikes have continued since Foley's killing and on Tuesday appeared to bear fruit, as Iraqi forces continued their fightback against the jihadists who have seized much of northern Iraq.

After breaking a months-long siege of the Shiite Turkmen town of Amerli, government troops also regained control of part of a key highway linking Baghdad to the north of the country.

Two towns north of Amerli had already been retaken on Monday as Iraqi forces -- backed by US strikes -- won their first major victories since the army's collapse across much of the north in June.

- 'Ethnic cleansing' -

The jihadists have reportedly carried out widespread atrocities. On Tuesday, Amnesty International accused them of war crimes and ethnic cleansing.

The Sunni extremist group has declared an Islamic "caliphate" in regions under its control in Iraq and Syria.

In June, it swept through much of the Sunni Arab heartland north of Baghdad and then stormed minority Christian and Yazidi areas.

The extremist faction has carried out beheadings, crucifixions and public stonings, and Amnesty accused it of "war crimes, including mass summary killings and abductions" in areas it controls.

Assistance is now arriving in Amerli, brought in both by Shiite militia fighters and the United Nations, which said it had "delivered 45 metric tonnes of life-saving supplies."

Westerners executed by jihadist groups
Washington (AFP) Sept 02, 2014 - Below is a list of previous executions of westerners by jihadist groups since 2002, after the reported death in Syria of American journalist Steven Sotloff whose beheading has been claimed by the ultra-radical Islamic State.

A propaganda video apparently showing the ghastly execution was released Tuesday, with IS stating the killing was in response to US air strikes against its positions in Iraq.

US officials were working to authenticate the footage, which if confirmed would mark the second IS video of a beheaded American in as many weeks.

Sotloff, well-versed in the history and culture of Islam and the Middle East, went missing August 4, 2013.

-- AMERICANS --

- January 23, 2002 - PAKISTAN: Reporter Daniel Pearl, working for The Wall Street Journal and researching a story about Islamist militants, is kidnapped in the southern city of Karachi and beheaded.

A graphic video showing his decapitation is delivered to the US consulate a month later.

- October 28, 2002 - JORDAN: Diplomat Laurence Foley, responsible for the US development agency USAID is shot at close range outside his Amman home.

Jordanian Abu Mussab Zarqawi, the head of the Al-Qaeda terror network in Iraq, is in 2004 sentenced to death in his absence for the killing. He is killed in 2006 in an American raid north of Baghdad.

- May 11, 2004 - IRAQ: A video shown on an Internet site linked to Al-Qaeda shows the beheading of businessman Nicholas Berg, who was abducted early April in Baghdad.

According to US security forces, the masked executioner is Zarqawi.

- June 18, 2004 - SAUDI ARABIA: An Islamist Internet site disseminates photos of the beheading of American aeronautical engineer Paul Marshall Johnson, who had been abducted several days earlier in Riyadh.

On August 19, 2014, a Riyadh court sentences an Al-Qaeda member to death and imposes jail terms of four to 30 years on 13 co-defendants for the crimes.

- September 20, 2004 - IRAQ: A particularly hardline group, led by Zarqawi, announces the decapitation of Eugene Armstrong, an American kidnapped September 16 in Baghdad, with one of his compatriots Jack Hensley and Briton Kenneth Bigley.

Hensley's execution is announced a day later.

- August 19, 2014 - IRAQ/SYRIA: Islamic State, the extremist group marauding across Iraq and Syria, posts a video of the decapitation of US journalist James Foley, 40, who was seized in northern Syria in November 2012.

One day later US officials announce there had been a failed earlier bid to rescue American hostages in Syria.

-- BRITONS --

- October 8, 2004 - IRAQ: The execution of engineer Bigley is announced, three weeks after his abduction by Zarqawi's group.

- June 3, 2009 - THE SAHEL: The North African branch of Al-Qaeda, AQIM, claims its first execution of a western hostage, Edwin Dyer, kidnapped on January 22 in the border area between Mali and Niger.

The tourist was in the hands of the group of Algerian Abdelhamid Abou Zeid, who several witnesses say cut Dyer's throat himself.

-- FRENCH --

- April 19, 2010 - NIGER: Michel Germaneau, a retired French engineer, 78, is abducted in northern Niger and taken to Mali by AQIM, which announces his execution on July 25 after a failed Franco-Mauritanian raid to liberate him. Seven members of AQIM are killed in the raid.

- November 24, 2011 - MALI: Philippe Verdon is kidnapped from his hotel in Hombori in northeastern Mali. On March 20, 2013, AQIM announces that it has killed Verdon, in response to France's intervention in the country. The autopsy reveals that he was shot in the head.

-- ITALIANS --

- April 14, 2004 - IRAQ: Italian security guard Fabrizio Quattrocchi is shot at point-blank range, two days after his abduction, to punish Italy for refusing to withdraw its soldiers from Iraq.

He becomes the first foreign hostage killed in Iraq.

- August 20, 2004 - IRAQ: Italian journalist Enzo Baldoni is abducted on the road between Baghdad and Najaf. His filmed execution is claimed August 26 by a group calling itself the Islamic Army in Iraq.

- April 15, 2011 - GAZA STRIP: Vittorio Arrigoni, a pro-Palestinian militant, is found hanged in a house in the Gaza Strip several hours after having been taken hostage by a salafist-jihadist group.

-- POLE --

- February 7, 2009 - PAKISTAN: Polish geologist Piotr Stanczak is beheaded after having been held for more than four months by Islamist militants in northwestern Pakistan.