Iraqi security forces have captured several areas on the outskirts of Fallujah on the first day of an offensive aimed at wresting the strategic city from Islamic State militants, military and security officials said on Monday.
Security forces moved into the Karma, Shihabi and al-Sejar areas west of the city in the early morning, hours after Prime Minister Haider al-Abadi announced the offensive, the officials said on condition of anonymity.
Al-Abadi's office published pictures of the premier, who is also the commander in chief of the armed forces, wearing military fatigues as he presided over a meeting of commanders at the operation's headquarters.
Al-Abadi said that operations were proceeding "successfully", adding that "there is a big challenge and that is preserving the lives of civilians."
Safe corridors had been established for civilians fleeing the city, he said, and those who were unable to leave should stay in their homes.
The United Nations was "very concerned" about the situation of civilians remaining in Fallujah, putting their number at 50,000, said Stephane Dujarric, spokesman for UN Secretary General Ban Ki-moon.
"One of the problems is that civilians are in great danger as they try to flee, and it's important that they have some safe corridors that they could use," Dujarric said.
He added that the UN was providing food, water and shelter to those who had fled the city, noting that rising temperatures have increased the risk of dehydration.
Fallujah is one of two main cities in the mainly Sunni al-Anbar province, which stretches west from Baghdad to the Syrian and Jordanian borders.
The province has been a key stronghold for the Sunni extremist group but security forces, backed by airstrikes by a US-led coalition, have made a series of major gains there in recent months.
Since December the militants have lost control of the provincial capital Ramadi, the city of Hit and, last week, the town of al-Rutbah near where the routes to Jordan and Syria meet.
Fallujah was one of the first major cities in Iraq to fall to Islamic State, in early 2014, when the jihadist group took advantage of local fury at a deadly crackdown on Sunni protests ordered by then prime minister Nuri al-Maliki.
Six months later, the jihadist group stormed through Sunni areas of northern Iraq, taking the country's second city Mosul as army units fled in disarray.
Al-Abadi replaced the hardline al-Maliki in September 2014, promising a more conciliatory approach to the Sunni minority.
Al-Abadi has vowed that Iraqi forces will recapture Mosul this year, although experts say Islamic State is well entrenched there and considerable preparation and training will be needed.