The US military's top officer, General Martin Dempsey, told lawmakers on Wednesday it was possible special operations forces could eventually be sent to Syria to back up American-trained rebels.

But officials said Dempsey's comment was addressing a "hypothetical" scenario for the moment as moderate opposition forces have not yet been trained.

Asked if more American troops would have to be sent to Iraq or Syria to take on the Islamic State group, Dempsey said if commanders requested it, he would be ready to endorse it to President Barack Obama.

Dempsey said "if the commander on the ground approaches either me or the secretary of defense and believes that the introduction of special operations forces to accompany Iraqis or the new Syrian forces," or forward air controllers, "these skilled folks who can call in close air support" are needed, "if we believe that's necessary to achieve our objectives, we will make that recommendation."

The general in the past has made similar comments referring to Iraq, where more than 2,000 US forces are deployed to train and advise Iraqi army troops and Kurdish forces.

The Obama administration has left open the possibility of sending in forward air controllers or more advisers to Iraq but not in Syria, where moderate rebels are due to be trained by US troops at sites in Turkey, Saudi Arabia and Qatar.

A defense official told AFP that Dempsey has not shifted his position and that "there is no consideration for sending US troops into Syria" beyond deploying rescue teams for downed pilots.

Dempsey was discussing a "hypothetical" question and addressing "flexibility and preservation of options" in a hearing about a proposed war powers law that would grant the US president authority to wage an open-ended campaign against the IS group, said the official, who spoke on condition of anonymity.