US strikes Islamic State in Iraq after hostage horror

The United States sought to maintain pressure on jihadist militants in Iraq on Thursday, launching air strikes after the murder of a journalist underlined the international threat posed by the Islamic State.

And, in the latest sign that a reluctant America could be dragged deeper into the fighting, officials revealed that US special forces had already carried out a failed hostage rescue mission inside Syria.

On Tuesday, the Islamic State released a video in which a militant with a British accent is seen beheading American journalist James Foley and threatening a second US hostage.

The murder has stoked fears in Britain and beyond that the territory that IS militants have seized in Syria and northern Iraq could become a launching pad for a new round of global extremist attacks.

US President Barack Obama called on countries in the region to join with the United States to "extract the cancer" of the Islamic State's jihadist ideology, and on Thursday US strikes continued.

In six bombings near a dam north of Mosul, US warplanes damaged three Humvee armored trucks, another vehicle and several roadside bomb "emplacements," Central Command said.

The US military has conducted 90 air strikes in Iraq since August 8, including the latest raids. Of those operations, 57 have been in support of Iraqi government forces near the Mosul dam.

Separately, officials confirmed that in recent months US special forces had carried out a raid inside eastern Syria to try to rescue people held hostage by IS militants, reportedly including Foley.

"This operation involved air and ground components and was focused on a particular captor network," Pentagon spokesman Rear Admiral John Kirby said.

"Unfortunately, the mission was not successful because the hostages were not present at the targeted location."

In the execution video -- released online by the Islamic State -- a black-clad militant said that Foley, a 40-year-old freelance journalist, was killed to avenge US air strikes against IS.

The man, speaking with a clear London accident, then paraded a second US reporter, Steven Sotloff, before the camera and said he, too, would die unless Obama changes course.

- 'Justice must be done' -

Foley was kidnapped in northern Syria in November 2012 and his grisly murder provoked global revulsion and condemnation.

On Thursday, US Attorney General Eric Holder, noted that the FBI had an open criminal probe into Foley's kidnap.

"We have long memories and our reach is very wide. We will not forget what happened and people will be held accountable -- one way or the other," he said.

On Wednesday, British Prime Minister David Cameron broke off his holiday to convene crisis meetings amid rising concerns about how many jihadists are walking Britain's streets.

"We have not identified the individual responsible, but from what we have seen, it looks increasingly likely that it is a British citizen," Cameron told reporters. "This is deeply shocking."

Interpol has called for a global response to the threat, with monitors covering the conflict in Syria saying the Islamic State has more than 50,000 fighters in that country alone, including 20,000 foreigners.

Interpol chief Ronald Noble said there should be a "multilateral response against the terror threat posed by radicalized transnational fighters traveling to conflict zones in the Middle East."

The scale of the threat became clear in June when the group, then known as the Islamic State in Iraq and the Levant, declared the dawn of a Muslim caliphate and seized the Iraqi city of Mosul.

Obama reacted this month by ordering US warplanes to counter threats to US personnel in the Kurdish regional capital Arbil and to civilian refugees from Iraqi religious minority groups.

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

Shiite militias, federal soldiers, Kurdish troops and Sunni Arab tribes have been battling IS for weeks in Iraq but have been unable to clinch a decisive victory, despite US air strikes.