HAQLANIYA, Iraq (Reuters) -- U.S. Marines fought their way into the insurgent stronghold of Haqlaniya on Wednesday as they intensified a campaign to bring the restive Iraqi Sunni Muslim province of Anbar under control.

A column of tanks and armored vehicles rolled into the town, about 150 miles west of Baghdad, before dawn and were immediately ambushed. Marines' forces responded with heavy machinegun fire and several tank rounds.

"We were hit by an IED (improvised explosive device), a daisy chain (three EIDs linked together) and then we took a rocket propelled grenade," said Sergeant Larry Long, speaking as shots and blasts echoed across the western desert.

U.S. forces this week launched a fresh push to tackle insurgents in Anbar, Iraq's huge western province, which stretches to the borders with Syria, Jordan and Saudi Arabia.

Sunni Muslim insurgents have built strongholds in towns and cities across Anbar. An offensive by U.S. troops on Falluja in November took the city back from guerrillas, but many fled to other locations such as Ramadi, Haditha, Hit and Haqlaniya, a town of 60,000.

"We're going into the city and we're staying," Lieutenant-Colonel Greg Stevens of the 1st Marines Expeditionary Force told his troops on the eve of the assault.

"The situation in Anbar has gone too far which is why we enacted River Blitz. We don't want to present a weak spot to the insurgent," he said, referring to this week's operation.

Haqlaniya has been a hotspot for months. Four Marines were killed in an ambush near the town in January.

The U.S. military said it was expecting strong resistance from foreign fighters who they say have links to al Qaeda. "The foreign fighters are crazy fanatics who want to martyr themselves," said Stevens.

A column of black smoke drifted across the town on Wednesday from a bomb blast as skirmishes continued during the morning.

Iraqi soldiers working with the U.S. forces were given the job of clearing Haqlaniya's two mosques.

Taking up positions in two schools, the U.S. forces planned to stay for around 24 hours while they interrogate suspects.

In Falluja last November they fought house-to-house to flush out insurgents, killing hundreds. The mission in Haqlaniya was designed to be more low-key.

"If someone is stupid enough to shoot at us, swing the hammer hard and swing it with conviction," Capt Chris DeAntoni told his unit before the mission got under way.

"But don't break things just for the sake of breaking things. I don't want to hear about war and how we can do what we want. We're there to make things better."

By Alister Bull