The top American military leader has told Congress that US ground forces could be deployed once more in Iraq.

Army General Martin Dempsey, the chairman of the Joint Chiefs of Staff, told a Senate panel that he would make the recommendation if President Barack Obama's campaign to defeat Islamic State (IS) militants fails.

"To be clear, if we reach the point where I believe our advisers should accompany Iraqi troops on attacks against specific ISIL targets, I will recommend that to the president," said Gen Dempsey, using another name for the terrorist group.

Pressed to expand, he said he "would go back to the president and make a recommendation that may include the use of ground forces".

He spoke after American warplanes stepped up the offensive against IS targets in Iraq, pounding targets southwest of Baghdad in two raids on Sunday and Monday.

Previous strikes had been of a defensive nature, while these were in direct support of Iraqi troops battling the militants.

Americans in Iraq are currently said to be serving purely in a combat advisory role to help Iraqi troops tackle the IS forces.

Gen Dempsey said that if Iraqi forces launched a major offensive to recapture Mosul, he might want US troops to accompany the Iraqi troops or provide close combat advice.

He also told senators the US was ready to strike the extremists in Syria.

"This will not look like 'shock and awe' because that is not how ISIL is organised," he said, "but it will be persistent and sustainable."

Gen Dempsey appeared alongside Defence Secretary Chuck Hagel, who warned the war would not be easy or brief.

"Victory is when we complete the mission of degrading, destroying and defeating ISIL," the Pentagon chief testified.

The Senate hearing was repeatedly disrupted by anti-war protesters.

Yesterday 30 countries agreed to "support the Iraqi government by any means necessary - including military assistance".

The British Prime Minister David Cameron has said Britain would seek United Nations support for any such plans.

He said the backing would be an important part of the blueprint for dealing with the extremists, who have seized swathes of territory in Iraq and Syria.

It is still unclear whether Britain will join in US airstrikes against IS in Syria, with ministers there refusing to rule it out but insisting that no decision had been made.