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."