AFP/Washington
The apparent beheading by jihadists of a US journalist sparked worldwide revulsion on Wednesday and drew pledges by outraged Western governments to aid those battling the militants.
In a video posted online Tuesday, a masked militant is shown beheading a man resembling James Foley, who has been missing since he was seized in Syria in November 2012.
The video also contains a threat from the Islamic State (IS) jihadist group to kill another reporter if US air strikes in Iraq continue.
The release of the video comes as the US air raids appear to have yielded some results, helping Kurdish and federal forces push IS fighters back from some recently-conquered areas in northern Iraq, including the strategic Mosul dam.
Europe's top powers swiftly condemned the jihadists, with France warning the world faced the "most serious international situation" since 2001.
British Prime Minister David Cameron cut short his holiday and rushed back to London, calling an urgent meeting to discuss how to deal with IS after the man filmed carrying out the apparent beheading had a British accent, prompting speculation he is a UK national.
In a highly significant move, Germany said it was ready to send weapons to support Iraqi Kurds in their battle against IS, while France vowed to hold a conference on the security of the region and the battle against the "barbaric" militants.
Rights group Amnesty International said the apparent execution-style killing was a "war crime".
Foley's mother Diane paid tribute to her son, in a Facebook message to supporters.
"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," she said.
"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 at 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.
- 'A message to America' -
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 in English.
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.
IS, 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.
According to Kurdish officers, another US air strike was carried out early Wednesday, targeting an apparent jihadist meeting at a school in the area of the dam. Washington did not immediately confirm the raid.
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.
In north Iraq, the UN refugee agency meanwhile said it has begun a "massive 10-day operation" to provide assistance to the hundreds of thousands of people displaced by violence.
