Islamic State militants have claimed to have executed American journalist James Foley in revenge for US air strikes against its fighters in Iraq.

The Islamist group released a video showing a masked militant purportedly beheading the reporter, who has been missing since he was seized by armed men in Syria in November 2012.

"Find James Foley," the campaign run by his family to secure the 40-year-old freelancer's release, posted a public message online following the release of the video asking for time "to seek answers."

Mr 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 for the Global Post, AFP and other outlets.

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

His family has not heard from him since, despite a public campaign for information.

In the five-minute video, distributed online by known Islamic State sources, the group declares that Mr Foley was killed after US President Barack Obama ordered air strikes against IS positions in northern Iraq.

The purported execution 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 with a British accent.

The militants have also claimed in a video to be holding US journalist Steven Sotloff and said his life depended on US President Barack Obama's next move.

"The life of this American citizen, Obama, depends on your next decision," said a masked man in the video posted on social media sites, speaking English with a British accent as he held a prisoner the video named as Steven Sotloff.

The video could not immediately be verified. 

Earlier, the Islamic State militant group that has seized large parts of Iraq warned the United States it will attack Americans "in any place" if US air strikes hit its fighters.

The video, which shows a photograph of an American who was beheaded during the US occupation of Iraq and victims of snipers, featured a statement which said in English "we will drown all of you in blood".

US air strikes in northern Iraq have helped Kurdish fighters take back some territory captured by IS, who have threatened to march on Baghdad.

The latest advance by IS, an al-Qaeda offshoot, sent tens of thousands of members of the Yazidi ethnic minority and Christians fleeing for their lives and alarmed the Baghdad government and its Western allies.

Unlike al-Qaeda, IS has so far focused on seizing land in Iraq and Syria for its self-proclaimed caliphate, and not attacks on Western targets.

US President Barack Obama said at a news conference yesterday that IS posed a threat to Iraq and the entire region.

US military aircraft have carried out 35 air strikes against IS militants in Iraq over the past three days, destroying more than 90 targets.

The strikes marked the most intensive US bombardments of IS positions since they began on 8 August.

Drones and fighter, bomber and attack jets "eliminated" Islamist fighters' positions as the militants battled for control of a major dam in northern Iraq, Pentagon spokesman Rear Admiral John Kirby said.

"In all, we destroyed over 90 targets including a range of vehicles, equipment and fighting positions," he added in a statement.

"Iraqi forces have cleared the dam and are working to further expand their area of control."

Earlier, US Central command indicated that US warplanes and drones carried out 15 air strikes yesterday alone.

Nine IS positions and eight vehicles around the Mosul dam, where insurgents are fighting Kurdish forces, were destroyed CENTCOM added.

US Defense Secretary Chuck Hagel said US forces stood ready to pursue more missions along those lines, according to Rear Admiral Kirby.

"Secretary Hagel also commends the way in which Iraqi forces worked together in this operation. It reflects the growing determination of Iraqis to fight back," the spokesman added.