US Senator John McCain on Monday urged the deployment of international ground forces to combat jihadists in Syria and Iraq, as he toured the Middle East with a Senate delegation.
"For months we've been bombing (Syrian border town) Kobane and we still haven't driven ISIS out," the Senate Armed Services Committee chairman told reporters in Jerusalem, using an acronym for the Islamic State group.
"Since the air campaign started... ISIS has increased their size and areas of control.
"The reality is, we need more boots on the ground... we need intelligence, we need special forces, and we can't treat Iraq and Syria as different battlegrounds because it's the same enemy," McCain said.
"We are going to have to have an aircraft exclusionary zone which provides a buffer in order for us to not only train and equip (rebels), but to take care of refugees," he added.
IS began its offensive on Kobane in mid-September and came close to overrunning the town. But Kurdish fighters, backed by a steady campaign of US-led air strikes, have gradually recaptured territory.
McCain was heading a delegation of US senators on a trip to the region that included visits to Israel, Qatar and Saudi Arabia.
The US military is to send more than 400 troops to train Syrian rebel forces as part of a long-planned effort to build up a moderate opposition to take on IS.
The training will take place in Saudi Arabia, Turkey and Qatar and is expected to start in the "early spring," the Pentagon says.
But President Barack Obama, wary of having the US drawn into the multi-sided civil war in Syria, has been accused by some rebel groups, lawmakers and allies of moving too slowly to help the Syrian opposition.
US troops have already started training Iraqi government and Kurdish forces in the fight against the IS jihadists, with about 2,100 Americans deployed to Iraq.