The US military is flying armed drones over Baghdad to defend American troops and diplomats in the Iraqi capital if necessary, officials said Friday.

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.

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

The flights involved "a few" drones and were ordered as a precaution to safeguard Americans in Baghdad, for what the military calls "force protection," he said.

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 Pentagon acknowledged that among the manned and unmanned aircraft flying over Iraq to carry out surveillance, some were carrying bombs and missiles -- without specifying if those planes were drones.

"The reason that some of those aircraft are armed is primarily for force protection reasons now that we have introduced into the country some military advisers whose objective will be to operate outside the confines of the embassy," spokesman Rear Admiral John Kirby told a press conference.

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, he said.

"The president has made no decisions about the use of kinetic force, but it would be irresponsible for us not to be planning, preparing and thinking and to be ready in case he should make that decision," Kirby said.

- US surveillance effort -

The US military advisers, drawn mainly form special operations forces, along with units sent to bolster security for the US embassy in Baghdad, bring the total number of American troops in the country 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 -- which was being carried out on Baghdad's request --- includes armed F-18 fighter jets, flying from the George H. W. Bush aircraft carrier in the Gulf.

American drones are not yet able to fly from carriers but the US military has the robotic aircraft on bases around the Middle East.

The Pentagon would not say where the drones over Iraq were being flown from, as Arab governments in the region are often reluctant to publicize their military cooperation with the United States.

After the Iraqi army suffered a succession of battlefield defeats at the hands of the Sunni extremists, the Obama administration has vowed to expedite weapons deliveries to Baghdad.

About 300 Hellfire missiles had been delivered at the beginning of the year, 200 were due to arrive in "mid-July," and another 600 by the end of July, Kirby said.

In addition, Baghdad had requested a purchase of an additional 800 Hellfire missiles as well as grenades and small arms ammunition.

The Pentagon press secretary also rejected accusations that Washington was seeking to delay the delivery of Iraq's first F-16 aircraft.

The first jets have long been scheduled to arrive later this year in the "fall" and the schedule has not changed, Kirby said.