The top US general told lawmakers on Thursday he would consider sending American military advisers to accompany Iraqi troops if they advance against Islamic State militants in the difficult terrain near Mosul and the Iraqi border with Syria.

Army General Martin Dempsey, the chairman of the Joint Chiefs of Staff, told a congressional hearing that he had not decided if such a recommendation would be necessary.

A spokesman said Dempsey was not referring to sending in ground combat troops to fight militants but was referring to deploying advisers to help call in air strikes, an idea he first raised publicly in September.

The general's remarks could stir political concern just days after Obama authorised expanding the number of US troops in Iraq to as many as 3,100 from a current limit of 1,600, adding more advisers as well as troops to train the Iraqi military.

Dempsey said he was "certainly considering" sending US advisers to support Iraqi troops. He spoke after members of the Republican-led House Armed Services Committee questioned why Democratic President Barack Obama had ruled out putting combat troops in Iraq and asked the general whether he would recommend greater action.

"How can you successfully execute the mission you've been given to degrade and ultimately destroy ISIL (Islamic State) when some of your best options are taken off the table?" asked Representative Buck McKeon, the committee chairman.

McKeon said if Obama sought a new authorisation to use military force but ruled out US combat troops, it would not be approved. "I will not support sending our military into harm's way with their arms tied behind their backs," he said.

Defence Secretary Chuck Hagel told the panel the US-led air campaign against Islamic State, which has been criticised for the small number of air strikes, would intensify as Iraqi forces strengthened to the point they could go on the offensive.

Dempsey and Hagel urged Congress to approve $5.6 billion in additional funding for the campaign.

The general told the panel Iraq needed 80,000 trained and competent troops to recapture Mosul, retake territory lost to Islamic State and control the border with Syria. "We're on the path to conduct that training," he said.

(Reuters)