Thousands of soldiers were advancing on Tikrit, which was overrun by insurgents led by the Islamic State of Iraq and the Levant (ISIL) more than two weeks ago along with a swathe of northern and north-central Iraq.
A senior US official, speaking on condition of anonymity, said "a few" armed drones were being used over Baghdad as a precaution to safeguard Americans, but they will not be used for offensive action against the militants.
The Pentagon confirmed that among the manned and unmanned US aircraft flying over Iraq to carry out surveillance, some were carrying bombs and missiles.
"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 said.
The US flights come despite the insistence of Prime Minister Nouri al-Maliki on Friday that "Baghdad is safe" from militant assault.
Maliki was backed in that view by retired US general James Conway, who said that "the worst is over" in the jihadist-led offensive, as the Sunni Arab militants would not be able to penetrate Baghdad or the predominantly Shia Muslim south or Kurdish north.
Taking back Tikrit
Although they initially wilted in the face of the offensive in majority Sunni Arab areas north and west of Baghdad, the security forces have appeared to perform more capably in recent days and on Saturday were readying what would be their biggest fightback so far.
Thousands of soldiers, backed by air cover, tanks and bomb disposal units, were advancing on now-executed dictator Saddam Hussein's hometown Tikrit, which fell to insurgents on June 11.
"A large military operation started today to clear Tikrit of ISIL," Staff Lt. Gen. Sabah Fatlawi told AFP, confidently adding: "ISIL fighters now have two choices — flee or be killed."
On Thursday, Iraqi forces swooped into into the city's strategically-located university campus by helicopter, with sporadic clashes reported throughout Friday.
Taking the university is seen as an important step towards regaining control of Tikrit, one of the biggest cities controlled by the militants.
World leaders have insisted, however, that a political settlement be reached between Iraq's Shia Arab, Sunni Arab and Kurdish communities alongside military action to see off the insurgent threat.
Top Shia cleric Sistani urged Iraqi leaders to unite and form a government quickly after the new parliament elected on April 30 convenes on Tuesday.
Maliki, who has publicly focused on a military response to the crisis, has acknowledged that political measures are also necessary.
US air strikes wanted
Iraq has appealed for US air strikes against the militants, but Washington has offered only up to 300 military advisers.
US officials have said that a $500 million plan to arm and train moderate rebels in neighboring Syria could also help Iraq's fight against ISIL, which operates on both sides of the border.
But amid calls for unity, Iraqi Kurdish leader Massud Barzani said Baghdad could no longer object to Kurdish self-rule in Kirkuk and other towns from which federal forces withdrew in the face of the militant advance.
"Now, this [issue] ... is achieved," he said, referring to a constitutional article meant to address the Kurds' decades-old ambition to incorporate the territory in their autonomous region in the north over the objections of successive governments in Baghdad.
Mortar fire south of Baghdad killed at least five people on Friday, while shelling and clashes in Diyala province to the northeast killed 10 more, four of them soldiers.
Maliki's security spokesman has said hundreds of soldiers have been killed since the offensive began, while the UN puts the overall death toll at nearly 1,100.
The International Organization for Migration warned that aid workers could not reach tens of thousands of Iraqis displaced by the violence, and called for humanitarian corridors to be established.