The killings came after the US on Wednesday launched its second major anti-resistance operation in Iraq's vast western region in less than a month.
The offensive is aimed at uprooting fighters who have killed more than 620 people since a new Iraqi government was announced on 28 April.
In the US offensive, helicopters swept down near palm tree groves, dropping off marines who blocked off one side of Haditha, while other troops on foot and in armoured vehicles established checkpoints and began moving towards the centre of the city, 220km northwest of Baghdad. US warplanes circled overhead.
Many of the fighters are thought to be foreign fighters who have slipped across the border from Syria.
Syria is under pressure to stop the alleged entry of foreign fighters into Iraq from the Syrian border.
Both the United States and Iraq, at their highest leadership levels, have been demanding Syria do more. Iraqi Prime Minister Ibrahim al-Jaafari said last week he would soon visit Syria for talks with officials on repeated border infiltration.
"We believe there are many infiltrators, many terrorists, that are stepping through the borders carrying out terrorist attacks against innocent Iraqis," Foreign Minister Hoshyar Zebari said at a joint news conference with visiting Italian Foreign Minister Gianfranco Fini.
Zebari said: "There are responsibilities of the Syrian government to hamper and prevent this flow of terrorists from coming across."
Zebari (R) held talks with Fini (L)
over troop deployment in Iraq
Operation New Market
"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 Lieutenant-Colonel Lionel Urquhart, commander of the 3rd Battalion, 25th Marine Regiment, which is part of the operation.
The assault, called Operation New Market, focussed on the city of about 90,000 people, where the US military says fighters have been using increasingly sophisticated tactics.
Iraqi journalist Nuri al-Tamimi told Aljazeera the US military operation was one of many on small Iraqi cities, and said there were no international observers or human rights groups to report on what was happening.This month, fighters launched a multistage attack from a Haditha hospital, killing four US troops in an ambush that included a car bomber, a roadside bomb and gunfire from fortified positions in the hospital, which was partly destroyed in the attack.
Haditha was the scene of a
resistance ambush this month
Al-Tamimi reported that US forces were killing Iraqis civilians and said fighters were equipped with simple weapons.
According to initial reports, three fighters were killed in gun battles that broke out after US forces entered the town before dawn, Marine Captain Christopher Toland told an Associated Press reporter, embedded with US forces.
Four US soldiers were killed on Tuesday, pushing the number of US troops killed in four days to 14, part of a surge in attacks that have also killed about 60 Iraqis.
Flushing out fighters
US marines took over several homes in Haditha, using them as observation and control centres, as other troops fanned out through the city's mainly empty streets in an apparent bid to flush out any fighters.
At least one loud explosion rocked the city early this morning, but the source of the blast was unclear.
"A lot of this is like bird hunting. You rustle it up and see what comes up"
Stephen W Davis,
US Marines colonel
The latest campaign demonstrates the military's ongoing concerns about fighters in both small and large cities in Sunni-dominated areas of the country.
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 Colonel Stephen 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, fighters fired a mortar at a hydroelectric dam facility near Haditha where hundreds of marines are based.
US officials said they hoped their presence would allow locals to feel safe enough to provide tips to the military.
Hundreds of marines based near
Haditha were attacked
"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.
Haditha lies along a major highway used by travellers moving from western Iraq to major cities, such as Mosul and Baghdad, in the central and northern parts of the country.
Ahmad al-Zawiti, Aljazeera's correspondent in the northern Iraqi city of Dahuk reported that a bomb exploded at 10am in front of a petrol station in Shandukh neighbourhood, on the highway linking Dahuk with Mosul.
No casualties have been reported, he added.
When police arrived at the scene, another bomb exploded near the same place, killing a traffic police officer and wounding seven, according to a source at the Dahuk police station.
There have been many attempts to disturb security in Dahuk, the safest province in Iraq, al-Zawiti said.
Police were unable to close checkpoints or border crossings, as many people travel as tourists to Dahuk from southern and central Iraq.