WASHINGTON (AFP) - The US military is flying "a few" armed drones over Baghdad to defend American troops and diplomats in the Iraqi capital if necessary, a senior US official said on Friday.

"For the last 24 to 48 hours, we've started that," the official, who spoke on condition of anonymity, told AFP.

The move comes after the United States deployed 180 troops as military advisers in recent days to help the Iraqi government army fend off the advance of Sunni militants, who have captured territory north and west of the capital.

But officials said the armed drones would not be used to carry out offensive strikes on Sunni extremists, a move that would require a decision by President Barack Obama.

The drones were there as a precaution to safeguard Americans in Baghdad, or what the military calls "force protection," officials said.

Obama has not ruled out air strikes but for the moment, American forces are focused on gauging the state of the Iraqi military and its adversaries on the battlefield, according to the White House and the Pentagon.

The US advisers, drawn mainly form special operations forces, along with troops sent in to bolster security for the US embassy in Baghdad, bring the total number of American military personnel to roughly 500, officials said.

The armed robotic planes are in addition to other manned and unmanned US aircraft that are conducting about 30-35 surveillance flights a day, as Washington attempts to gain a better picture of events on the ground.

The surveillance effort includes armed F-18 fighter jets, flying from the George H. W. Bush aircraft carrier in the Gulf.