BAGHDAD — Iraqi forces have begun an assault on Falluja, a city that has been held by the Islamic State longer than any other in Iraq or Syria, Prime Minister Haider al-Abadi said in a televised speech on Monday.
“Today, we will tear down the black flags of the strangers who have kidnapped this city,” Mr. Abadi said, referring to the flags of the Islamic State that have been flying in Falluja for more than two years, in a speech just after midnight, alongside military commanders.
By daybreak, it did not appear that forces had begun entering the city or that mortar and artillery fire was being fired on the city from afar. That had been the case for months, as army units and Shiite militias, many of which operate outside the control of the Iraqi government and answer to Iran, have laid siege to the city.
Mr. Abadi and other Iraqi leaders have frequently made bold statements heralding new military offensives, only for the efforts to stall on the ground.
Any ground battle to liberate Falluja, often called the “city of mosques” and a stronghold of Sunni extremism — it was something of a birthplace for the Islamic State’s predecessor, Al Qaeda in Mesopotamia — is likely to be long and bloody.
The United States military, led by Marines, fought two battles for the city in 2004, and the urban, house-to-house fighting was some of the toughest the American military had faced since the Vietnam War.
Fighters of the Islamic State, also known as ISIS or ISIL, have held the city since the beginning of 2014. They are believed to be deeply entrenched and likely to stay and fight, unlike in cities like Tikrit and Ramadi, which they eventually fled as Iraqi security forces closed in.
Perhaps mindful of the difficulty the United States faced in pacifying Falluja, American officials have mostly urged the Iraqis to refrain from trying to take back the city.
Instead, they have pressed the Iraqis to focus on other targets, such as Mosul, one of the country’s largest cities, which has been in the hands of the Islamic State since June 2014.
Backed by American and coalition airstrikes, Iraqi forces have made progress this year in liberating territory held by the Islamic State, including Ramadi, the capital of Anbar Province, and other cities in the Euphrates River Valley.
But they have come at a steep cost in the level of destruction, and military victories have not been matched by any sense of reconciliation between Sunni and Shiite Arabs that would suggest a peaceful future for Iraq.
For the Iraqis, momentum for a Falluja offensive gained steam late last week after a surge in Islamic State attacks inside Baghdad, which killed nearly 200 people.
Targeting Falluja, only 40 miles from the capital, was immediately seen by the government and militia leaders as necessary to protect Baghdad, and also as a way to exact revenge.
“The moment has come to liberate a city in the land of Iraq,” the Iraqi military said in a statement, “the land that will never accept humiliation because the people of this country have determined to end the darkness of terrorism of ISIS criminal gangs.”
The announcement by Mr. Abadi came after the authorities asked civilians on Sunday to prepare to leave the city, promising that safe passage would be provided.
For Mr. Abadi, there is also a potential political benefit to beginning the offensive, or at least to announcing the start of one: it allows him to convey a sense of authority at a time when his government is paralyzed and facing growing street protests.
On Friday, for the second time in three weeks, protesters breached the heavy security of the Green Zone, the fortified center of Baghdad that houses government buildings and embassies.
Protesters even got inside Mr. Abadi’s own office before withdrawing, but not before at least two died in the unrest, as security forces fired tear gas and live ammunition.
For months, militias and Iraqi security forces have surrounded Falluja, putting the city under a siege that has led to starvation and shortages of medicine.
Tens of thousands of civilians remain inside and would be unable to leave even if they wanted to. The Islamic State would shoot them on the way out, and militias on the outskirts of the city sometimes refused to allow civilians to pass, human rights activists have warned.
The city fell to the Islamic State nearly two and a half years ago, after the Iraqi Army attacked a protest encampment where Sunnis had been gathering for nearly a year to express their grievances toward the previous government of the prime minister at the time, Nuri Kamal al-Maliki, whose sectarian policies have been blamed by the United States for the rise of the militant group.