Jihadists released a video apparently showing the beheading of an American journalist kidnapped in Syria, in the most direct retaliation yet to US air strikes against them in Iraq.

The video, in which the Islamic State (IS) jihadist group also threatens to kill another reporter if US air strikes in Iraq continue, sparked global outrage on Wednesday.

Posted online late Tuesday, it shows a masked militant beheading a man resembling James Foley, who has been missing since he was seized in Syria in November 2012.

News of Foley's apparent beheading comes as US air strikes appeared to yield some results, helping Kurdish and federal forces push IS fighters back from some recently-conquered areas in northern Iraq, including the strategic Mosul dam.

According to Kurdish officers, another US air strike was carried out early Wednesday, targeting an apparent jihadist meeting at a school near the dam. Washington did not immediately confirm the raid.

In a Facebook message to supporters, Foley's mother Diane said: "We have never been prouder of our son Jim. He gave his life trying to expose the world to the suffering of the Syrian people.

"We implore the kidnappers to spare the lives of the remaining hostages. Like Jim, they are innocents. They have no control over American government policy in Iraq, Syria or anywhere in the world."

The White House said US intelligence was studying the video, and that President Barack Obama had been briefed on it as he flew from Washington to resume his vacation on Martha's Vineyard.

"If genuine, we are appalled by the brutal murder of an innocent American journalist and we express our deepest condolences to his family and friends," National Security Council spokeswoman Caitlin Hayden said.

Foley was an experienced correspondent who had covered the war in Libya before heading to Syria to follow the revolt against Bashar al-Assad's regime, contributing to news site GlobalPost, Agence France-Presse (AFP) and other media outlets.

AFP chairman Emmanuel Hoog described Foley "as a brave, independent and impartial journalist" whose work in Syria and other war zones was "widely admired".

According to witnesses, Foley was seized in the northern Syrian province of Idlib on November 22, 2012.

British accent

In the nearly five-minute video, titled "A Message to America," IS declares that Foley was killed because Obama ordered air strikes against IS in northern Iraq.

The beheading is carried out in an open desert area with no immediate signs as to whether it is in Iraq or Syria by a black-clad masked militant who speaks English with a British accent.

Foley is seen kneeling on the ground, dressed in an orange outfit that resembles those worn by prisoners held at the US naval base at Guantanamo Bay.

"Any aggression towards the Islamic State is an aggression towards Muslims from all walks of life who have accepted the Islamic caliphate as their leadership," the masked militant declares.

He threatens to kill another man shown in the video and said to be Steven Sotloff, whose kidnapping in August 2013 has not been widely reported.

He has written for several US newspapers and magazines, including Time, Foreign Policy and The Christian Science Monitor.

France on Wednesday slammed the apparent killing as "a barbaric act that plays on fear," while British premier David Cameron termed it "shocking and depraved" as he cut short a holiday.

Germany, denouncing the jihadists as "barbaric", said it would send arms to Iraqi Kurdish forces battling them.

The group, which led an offensive that has overrun large chunks of Iraq and neighbouring Syria, has declared a cross-border caliphate -- a successor state to historic Muslim empires.

Expanded strikes

Earlier this month, Obama reacted by ordering US warplanes to strike the jihadists, arguing they threatened US personnel in the Kurdish regional capital Arbil and risked carrying out a genocide against religious minority groups.

Obama has insisted the scope of the strikes would remain limited but Iraqi officials and observers have argued only foreign intervention could turn the tide on jihadist expansion in Iraq.

Shiite militia, federal soldiers, Kurdish troops and Sunni Arab tribes have been battling IS for weeks in some areas but have been unable to clinch a decisive victory.

An offensive launched on Tuesday against Saddam Hussein's hometown Tikrit was presented as a major push to liberate the city, but it appeared to have stalled a few hours later.

Shellfire periodically hit the city on Wednesday while Iraqi security forces remained positioned outside it, police and witnesses said.

US and Iraqi officials say the strikes have already had an impact on IS morale but the intervention may also have galvanised some fighters as fighting Americans is a source of prestige in global jihad.