THE top US military leader has said he would recommend that the US consider deploying ground forces to Iraq if President Barack Obama's expanded air campaign to destroy Islamic extremists fails.

Army general Martin Dempsey, the chairman of the Joint Chiefs of Staff, told a Senate panel that the goal for American advisers is to help Iraqi forces with planning, logistics and co-ordinating military efforts by coalition partners to take out members of the Islamic State (IS) group.

"To be clear, if we reach the point where I believe our advisers should accompany Iraqi troops on attacks against specific Isil (IS) targets, I will recommend that to the president," Gen Dempsey told the Senate armed services committee.

Pressed during questioning, Gen Dempsey said that under certain circumstances he "would go back to the president and make a recommendation that may include the use of ground forces".

Barack Obama has maintained that American forces will not have a combat mission in Iraq. Responding to Gen Dempsey's comments, the White House said the President's military advisers had to plan for many possibilities and that overall policy had not changed.

It came as the US House of Representatives began debating legislation to authorise Mr Obama's plan to arm and train moderate Syrian rebels to fight Islamic State militants. House members are expected to vote to pass the amendment today. It would then be sent to the US Senate for approval this week.

Meanwhile, a man who claims to have defected from Islamic State said the militants would respond to any intensification of Western air strikes by beheading more hostages.

The unnamed man, whose identity could not be independently verified, told BBC Radio 4's World at One the West was "stupid" in its outraged responses to IS's actions, which he said only encouraged the group to carry out more atrocities.

He said that the UK national nicknamed "jihadi John" was chosen to carry out the murders of Perth-raised aid worker David Haines and two US journalists in order to "send a message" to Britain, which IS leaders believe may join the US in military action. And he said other Britons were working through the internet to recruit more Westerners to swell the group's ranks.

The man told the BBC he was a Syrian fighting with another opposition group when the city of Raqqa was taken by IS. He joined up and was impressed by the extremists' efficiency and the "five-star lifestyle" they delivered their supporters, but decided to leave when he became disgusted by their brutality, including the arrest and torture of some of his friends.

Speaking through a translator, the man told World at One: "It is clear that the West are stupid. Whenever IS does something and the West gives speeches against it, IS becomes more determined to do similar things.

"If air raids intensify in Iraq and Syria, there will be more hostages beheaded."

Describing the activities of IS members, the man said: "Foreigners, like British and French fighters, are mainly recruiters of new people. They are in contact with the internet 24 hours a day trying to recruit new people on the borders. There was one person who was able to recruit three or four new fighters every week."