HADITHA, Iraq - About 1,000 U.S. Marines, sailors and soldiers encircled this Euphrates River city in the troubled Anbar province before dawn today, launching the second major anti-insurgent operation in this vast western region in less than a month.

The offensives are aimed at uprooting insurgents who have killed more than 620 people since a new Iraqi government was announced on April 28. Many of those insurgents are thought to be foreign fighters who have slipped across the border from Syria.

In Haditha, helicopters swept down near palm tree groves dropping off Marines who blocked off one side of the town, while other troops on foot and in armored vehicles established checkpoints and moved toward the center of this city, 140 miles northwest of Baghdad. U.S. 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 U.S. military says insurgents have been using increasingly sophisticated tactics. Earlier this month insurgents launched a multistage attack from a Haditha hospital, killing four U.S. 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 several fierce gun battles that broke out after U.S. forces entered this town before dawn, Marine Capt. Christopher Toland told an Associated Press reporter embedded with U.S. forces. Two Marines were also wounded and evacuated, Toland said.

U.S. 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.

The latest campaign demonstrates the military's ongoing concerns about insurgents in both small and large cities in Sunni-dominated areas of the country where large U.S. operations are still necessary to clear populated areas.

Haditha has no functioning police force, and U.S. 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 made of troops in Marine Regimental Combat Team 2.

A small reconnaissance unit of Iraqi soldiers is participating in the attack, Urquhart added.

Shortly before the assault began, insurgents fired a mortar at a hydroelectric dam facility near Haditha where hundreds of Marines are based.