Falluja assault: Iraqi military moves against IS
Clashes have been reported near the Iraqi city of Falluja hours after Prime Minister Haider al-Abadi announced the start of a military operation to retake it from so-called Islamic State (IS).
One report said the IS "governor" in Falluja had been killed, while the jihadist group said it had inflicted casualties on government forces.
"Zero hour for the liberation of Falluja has arrived," Mr al-Abadi said.
Falluja has been held by IS longer than any other in Iraq or Syria.
The Iraqi government has urged civilians there - estimated to number between 60,000 and 90,000 - to flee or raise a white flag over their homes.
Analysis: Jim Muir, BBC News, Baghdad
Falluja has been attacked many times and bombed and shelled almost incessantly since it fell into the militants' hands in January 2014. It has withstood all that, despite huge destruction and many casualties.
Now the government has committed itself to "liberating" the city once and for all, in an operation codenamed "Break Terrorism".
But there are conflicting assessments of how tough the battle will be.
Some believe that IS has taken such a pounding in the town that its ability to resist has been sapped. Others, in touch with sources inside the beleaguered city, say the militants have long been preparing to face such an offensive and have deployed their full array of defences, including many roadside bombs and booby traps.
Iraqi officials say corridors will be provided for civilians to make their way to camps outside the city, which was seized by IS militants and allied Sunni tribesmen in January 2014.
Iraqi TV said the IS wali, or "governor", in Falluja, Hejji Hamza, and some of his aides had been killed in the latest fighting. There were also reports of more than 30 IS fighters being killed outside the city.
Residents said air strikes and mortar attacks occurred overnight.
The IS-linked Amaq news agency said one of the group's suicide bombers had destroyed a tank and army bulldozer, killing 16 people.
Troops, police, militiamen and tribal fighters virtually surround the city, about 65 km (40 miles) west of Baghdad.
The Baghdad-based pro-Kurdish news website Shafaq said on Sunday that close to 20,000 police troops had arrived on the outskirts of Falluja ahead of the expected assault.
IS militants launched a sweeping offensive in June 2014, overrunning large areas north and west of Baghdad. However, security forces and allied fighters have pushed the jihadists back with support from US-led air strikes.
Iraq's military recaptured the nearby city of Ramadi, the capital of Anbar province, from IS in December.
A former US ambassador to Iraq predicted the Falluja operation would take some time.
"I think Haider al-Abadi has taken the advice of many of his American advisers, which was not to rush into this thing," Christopher Hill told the BBC.
"So they're being very deliberate, very careful, and so I think there's some good reason for optimism that ultimately this will be successful."
A former US military adviser in Iraq, Michael Pregent, questioned the wisdom of urging civilians to raise white flags.
"The problem with them [the Iraqi military] saying, 'raise a white flag so we don't shoot you' - IS is going to shoot them when they raise a white flag."
Should IS lose Falluja, it would leave the northern city of Mosul, Iraq's second-largest city, as its only major stronghold in Iraq. It continues to control large parts of Syria, including the city of Raqqa.
Last month, the United Nations and Human Rights Watch (HRW) warned civilians still living in Falluja were in danger of starvation.
The UN World Food Programme (WFP) said stocks were dwindling as government forces trying to recapture the city had cut supply routes, and IS had stopped people from leaving.
Some residents were eating grass to survive, HRW said.
One report said that a 110lb (50kg) bag of flour, which costs about $7.50 in the US, had been sold for as much as $4,166 (£2,925; €3,650).