The U.S. military is preparing for a spring offensive against Sunni insurgents and Al Qaeda fighters in the lawless city of Ramadi as part of the troop surge to impose security in Iraq.

U.S. and Iraqi security forces are currently performing a security crackdown in Baghdad, where they have already killed more than 400 suspected militants, according to Iraqi prime minister Nouri al-Maliki.

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With the battle to secure the capital under way, the fight for control of Ramadi in Anbar province to the west will mark the second phase of the surge. The group Al Qaeda in Iraq has exerted a grip on the city and terrorised sheikhs cooperating with the Americans.

President George W. Bush promised to send an extra 4,000 marines to Sunni-dominated Anbar when he announced the troop increase last month. According to defence sources, the battle for Ramadi is likely to begin around April, when the marines will be ready in strength.

“Ramadi is the last place to be sorted out,” said a senior defense source. “A lot of the sheikhs are already onside and it will be a much more straightforward operation than the battle for Baghdad.”

Dan Goure, a military analyst at the Lexington Institute in Virginia, said the Americans wanted to signal that they were back in force in the war-ravaged city.

There were clashes in Ramadi last week when Sunni insurgents armed with rocket-pro-pelled grenades attacked U.S. forces. Twelve Iraqis, including women and children, were killed in a six-hour firefight. Late last month a suicide bomber killed 16 people when he detonated a lorry filled with chlorine and explosives in Ramadi — an example of the insurgents’ newest fear-inducing method of attack.

Two more suicide bombers attacked a house belonging to Sheikh Sattar al-Buzayih, a tribal leader, last week, killing 11 in his entourage.

Buzayih, a prominent supporter of U.S. efforts to clear Al Qaeda out of the city, said recently that Anbar should be rid of terrorists who would “try to engineer our future with mortars and roadside bombs”.

General David Petraeus, the U.S. commander in Iraq, is hoping to persuade former Ba’athist generals to participate in expelling Al Qaeda from Ramadi.

“If they can enlist the generals, the hope is that the colonels and the majors will also join and it will bring a real coherence to the structure of the Iraqi forces,” said Dov Zakheim, a former senior Pentagon official.

According to defence sources, however, few generals from Saddam Hussein’s army have shown much interest in the task, despite financial inducements The Pentagon is divided over the wisdom of this tactic. The first battle for the Anbar town of Falluja in 2004 collapsed when a former Ba’athist general was put in charge and many of the soldiers under his command fled or defected to the insurgents.

Clearing out insurgents from Ramadi would present an explicit challenge to the Sunni community to support the American and Iraqi government crackdown or face the consequences. According to Goure, U.S. forces are inevitably “tilting” towards the Shi’ites and taking on the Sunnis in battle, despite their attempt at even-handedness.

Many Shi’ite militia leaders have fled Baghdad and their supporters have been ordered to lie low. Sunni insurgents in the capital have also been biding their time, while probing for signs of weakness the new “joint security stations” or “mini forts” established by U.S. troops in the heart of the city.

Petraeus’s plan for retaking control of Baghdad puts American forces at greater risk of attack than when they were stationed in large bases on the periphery, but the crackdown has sharply reduced the number of civilian deaths in the capital.

In Yarmouk, a largely Sunni neighbourhood just a mile from the fortified green zone, three new checkpoints were set up last week and every car was stopped. In a marked change of policy, Iraqi soldiers confiscated any weapons carried by civilians.

“Before the security plan started, we would find about 10 dead bodies every day,” said one checkpoint guard. “Nowadays we find about two bodies a day.”

Iraq closed its border crossings with Iran and Syria last week and set up new checkpoints in the hope of slowing the supply of arms and foreign fighters into the country.

Last week’s announcement of a cut of 1,600 in the number of British troops by May — when Prince Harry and his squadron from the Blues and Royals are expected to arrive in Iraq — was limited by the Americans’ insistence that Britain continue to “secure the Iranian border”.

If the troop surge succeeds in damping down the violence, the Americans will be tempted to declare victory and withdraw from Iraq, said Goure. “We’ll take the first opportunity of quiet to run like hell.”