BAGHDAD (AFP) - Iraqi forces pressed a campaign on Saturday to retake militant-held Tikrit, clashing with Sunni fighters nearby and pounding positions inside the city with air strikes in their biggest counter-offensive so far.
A senior officer said security forces were coordinating with Washington, which has deployed military advisers to help repulse the jihadist-led militants who have overrun large parts of five provinces north and west of Baghdad.
Armed US drones were flying over the city to provide protection for the advisers and US diplomats against the militants led by the jihadist Islamic State in Iraq and Syria (ISIS).
Top Shiite cleric Grand Ayatollah Ali al-Sistani, who is revered among Iraq's majority community, has urged political leaders to unite and form a new government to tackle the crisis.
International agencies have sounded the alarm over the humanitarian consequences of the fighting, with up to 10,000 people having fled a northern Christian town in recent days and 1.2 million displaced in Iraq this year.
Thousands of soldiers, backed by air cover, tanks and bomb disposal units, were advancing on Tikrit - executed dictator Saddam Hussein's hometown - which fell to insurgents on June 11.
Witnesses said troops were moving in from the west amid heavy clashes.
"A large military operation started today to clear Tikrit of ISIS," Staff Lieutenant General Sabah Fatlawi told AFP, saying that it has two choices - "flee or be killed".
Helicopter-borne troops landed at a strategically located university campus in the city on Thursday, with sporadic clashes reported throughout Friday.
Taking the university would be an important step towards retaking Tikrit, one of the largest militant-held cities.
Warplanes were targeting insurgents in the city, Prime Minister Nuri al-Maliki's security spokesman said.
Security forces also now fully controlled a key road from Baghdad to Samarra, between the capital and Tikrit, Lieutenant General Qassem Atta added.
There is coordination with the US over "studying important targets", Atta said, without elaborating.
Twenty security personnel - nine soldiers and 11 police - were killed in clashes with militants on Saturday in three areas southwest of Baghdad, an officer and doctors said.
Despite initial setbacks when the offensive in majority Sunni Arab areas began on June 9, the security forces have appeared to perform more capably in recent days.
A senior US official, speaking on condition of anonymity, said "a few" armed drones were being used over Baghdad to safeguard Americans, but they will not be used for offensive action.
The Pentagon confirmed that among the manned and unmanned US aircraft carrying out surveillance over Iraq, some carried bombs and missiles.
The flights come despite Maliki's insistence on Friday that "Baghdad is safe".
World leaders have insisted on a political settlement among Iraq's Shiite Arab, Sunni Arab and Kurdish communities.
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.
On Saturday, Russian Deputy Foreign Minister Sergei Ryabkov said in Damascus: "Russia will not remain passive to the attempts by some groups to spread terrorism in the region."
"The situation is very dangerous in Iraq and the foundations of the Iraqi state are under threat," he said.
Baghdad has agreed to buy more than a dozen Sukhoi warplanes from Russia and Belarus in a deal that could be worth up to US$500 million.
Iraq has appealed for US air strikes against the militants, but Washington has offered only up to 300 military advisers.
US officials have said a proposed US$500-million plan to arm and train moderate rebels in neighbouring Syria could also help Iraq fight ISIS, which operates in both countries.
Despite unity calls, Iraqi Kurdish leader Massud Barzani said Baghdad could no longer object to Kurdish self-rule in Kirkuk and other areas from which federal forces withdrew as the insurgents advanced.
"Now, this (issue)... is achieved," he said, referring to a constitutional article meant to address the Kurds' decades-old ambition to incorporate more territory into their autonomous northern region, which Baghdad opposes.
Maliki's security spokesman has said hundreds of soldiers have been killed since the offensive began on June 9, while the UN puts the overall death toll at nearly 1,100.
The International Organisation for Migration warned that aid could not reach tens of thousands of displaced Iraqis, and called for humanitarian corridors.