By Ross Colvin

BAGHDAD, March 30 (Reuters) - A surge of violence in Iraq in the past week demonstrated the ability of al Qaeda to strike virtually anywhere at will with a seemingly limitless supply of explosives and suicide bombers to wreak chaos.

The bombings claimed 300 lives, with one attack triggering mass reprisal killings by Shi'ites, making it the bloodiest week since the launch of a major U.S.-backed security crackdown in Baghdad in mid-February aimed at curbing sectarian violence.

The top U.S. commander in Iraq, General David Petraeus, accused Sunni Islamist al Qaeda on Friday of barbarity and said it was trying "to ignite sectarian violence" between minority Sunnis and majority Shi'ites and derail efforts to unify Iraqis.

Amid fears the country is being dragged ever closer to the brink of all-out civil war, Iraqi Prime Minister Nuri al-Maliki called for restraint, urging Iraqis not to allow themselves to be divided by "evil doers".

Crucially, the movement of radical Shi'ite cleric Moqtada al-Sadr said on Friday it would continue to give its backing to the crackdown in Baghdad. Sadr's Mehdi Army militia, accused by Washington of being the biggest threat to peace in Iraq, has kept a low profile since the launch of the operation.

Shi'ite residents in Baghdad areas that have been hit by bomb attacks in recent weeks say the absence of the Mehdi Army from the streets has given al Qaeda greater freedom to launch attacks and have called for the return of the militiamen to protect them.

"There is no alternative from the Baghdad security plan except anarchy. We in the Sadr bloc will continue to support the plan until we have security and order," the head of the bloc in parliament, Nassar al-Rubaie, told Reuters.

As thousands of Iraqi and U.S. troops have clamped down in Baghdad, the epicentre of the sectarian violence, and stepped up efforts to crush al Qaeda, the Sunni Islamist group has replied by unleashing what Petraeus called "sensational events".

CAR BOMBS

Suspected al Qaeda insurgents have struck police, Shi'ite civilians, U.S. soldiers and even a leader of a rival Sunni insurgent group this week in a series of quickfire, coordinated bomb attacks in western and northern Iraq and in and around Baghdad. Almost all have been carried out by suicide attackers.

Iraqi and U.S. officials blamed al Qaeda for the bombing of the revered Shi'ite shrine in Samarra in February 2006 that triggered a wave of reprisals by Shi'ite militias who until then had shown restraint. Tens of thousands of people have been killed in sectarian violence over the past few years.

Petraeus said al Qaeda was seeking to "undermine recent Iraqi and Coalition successes in improving security in Baghdad".

The security crackdown has been successful in reducing the number of daily shootings, although the number has started to creep up again from below 10 a day to the mid-20s. Before the operation started, police reported finding 40-50 bodies a day.

U.S. commanders concede they have had less success in curbing the bombings, despite the discovery of a number of car bomb factories and the killing of scores of al Qaeda militants. There has also been an upsurge in violence outside Baghdad.

The Brookings Institution, which monitors violence in Iraq, said there had been a 30 percent increase in Diyala province north of Baghdad since the launch of the crackdown in February. U.S. commanders there have asked for more reinforcements.

In one of the worst attacks this week, two truck bombs killed 85 people in a Shi'ite area of Tal Afar in northern Iraq. In the hours after those blasts Shi'ite gunmen, including police, shot dead up to 70 Sunni Arab men in swift reprisal.

Only a year ago U.S. President George W. Bush held up Tal Afar as a beacon of hope for Iraq after al Qaeda militants were ousted in a U.S. offensive a year earlier.

While analysts have offered various, mostly bleak, scenarios for Iraq, the savagery of the violence in Tal Afar could be a glimpse into the future.

(Additional reporting by Mussab Al-Khairalla in Baghdad)

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