The offensives are aimed at uprooting insurgents who have killed more than 620 people since a new Iraqi government was announced on April 28.
Helicopters swept down near palm tree groves, dropping off Marines who blocked off one side of Haditha, while other troops on foot and in armored vehicles established checkpoints and began moving toward the center of this city, 140 miles northwest of Baghdad. US warplanes circled overhead.
"Right now there's a larger threat than should be in Haditha, and we're here to tell them that they're not welcome," said Lt. Col. Lionel Urquhart, commander of the 3rd Battalion, 25th Marine Regiment, which is part of the operation.
The assault, called Operation New Market, focused on this city of about 90,000 people, where the US military says insurgents have been using increasingly sophisticated tactics.
Earlier this month insurgents launched a multistage attack from a Haditha hospital, killing four US troops in an ambush that included a suicide car bomber, a roadside bomb, and gunfire from fortified positions in the hospital, which was partially destroyed in the attack.
According to initial reports, three insurgents were killed during fierce gun battles that broke out after US forces entered this town before dawn, Marine Capt. Christopher Toland told an Associated Press reporter embedded with US forces.
Two Marines were also wounded and evacuated, Toland said.
Marines took over several homes in Haditha, using them as observation and control centers as other troops fanned out through the city's mainly empty streets in an apparent bid to flush any insurgents out.
At least one loud explosion rocked the city early this morning, but the source of the blast was unclear.
US military officials said the operation is about the same size as a weeklong assault dubbed Operation Matador that began May 7 and targeted insurgent outposts near the Syrian border, where nine Marines and an estimated 125insurgents were killed.
The latest campaign demonstrates the military's ongoing concerns about insurgents in both small and large cities in Sunni-dominated areas.
Haditha has no functioning police force, and US military officials acknowledge that their presence has been light in the city but say Iraqi troops are expected to arrive soon.
"A lot of this is like bird hunting. You rustle it up and see what comes up," said Marine Col. Stephen W. Davis, commander of the operation.
Shortly before the assault began, insurgents fired a mortar at a hydroelectric dam facility near Haditha where hundreds of Marines are based.
"Hold on, we'll be there in a minute," yelled Marine Sgt. Shawn Bryan, of Albuquerque, N.M., assigned to the 3rd Marine Battalion, from a platform on the dam as Marines scrambled into vehicles to try to locate the attackers.
Later, groups of soldiers and Marines huddled into groups and shouted prayers over water rushing from the dam into the Euphrates River shortly before they departed for the city.
US officials said they hoped their presence would allow locals to feel safe enough to provide tips to the military.
"The people out there know who wrecked the hospital and those who target their power source," said Urquhart, referring to the dam that is said to provide about a third of Iraq's electricity.
Several other attacks have occurred in Haditha this year, including the April 17 killing of a police chief and the discovery three days later of the bodies of 19 fishermen. US military officials say it's unclear if the fishermen were killed in a tribal dispute or by insurgents.
Haditha lies along a major highway used by travelers moving from western Iraq to major cities such as Mosul and Baghdad in the central and northern parts of the country.