Ten thousand American soldiers today launched one of the biggest operations in Iraq since the US-led invasion four years ago. Armoured columns and helicopter gunships began an offensive against suspected al-Qaeda strongholds in Diyala province north and east of Baghdad.

The operation, codenamed “Arrowhead Ripper”, was described by the US military as a “large-scale effort to eliminate al-Qaeda in Iraq terrorists” operating in the regional capital Baquba and outlying areas. In the early stages of the offensive, the US military claimed that 22 insurgents were killed.

“The end state is to destroy the al-Qaeda influences in this province and eliminate their threat against the people,” said Brigadier-General Mick Bednarek, the deputy commander of operations for the 25 Infantry Division.

“That is the number one, bottom-line, up-front, in-your-face, task and purpose,” he said.

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The American attack on the al-Qaeda stronghold came as fresh sectarian violence hit Baghdad. The Shia Khillani mosque was damaged by a truck bomb , which killed at least 75 people in the capital.

The Americans believe that they are up against some 2,000 fighters who have relocated in Diyala from across the country.

US troops are operating in difficult, agricultural terrain with small villages and hamlets scattered among palm trees, fields and irrigation ditches. The province is mainly a mix of Sunni and Shia Muslims and has been the scene of sectarian violence over the past year.

After facing attack by US-backed Sunni tribesmen in western Anbar province, al-Qaeda fighters, made up of local insurgents and foreign volunteers, have moved to Diyala over the past few months. It is here that Abu Musab al-Zarqawi, the former al-Qaeda leader in Iraq, was killed in a US air strike last year. But many of his supporters remain. In some areas al-Qaeda has openly paraded through the streets, enforcing strict Muslim laws, such as a ban on smoking, punishable by prison sentences in al-Qaeda’s own makeshift jails.

The focus of today’s operation is Baquba, a once-sleepy market town now seen as one of the most dangerous places in Iraq, where several hundred al-Qaeda fighters are thought to be based.

US troops began their assault into the western half of the city in the early hours of the morning. Once the area is secured, they will hand over control to Iraqi forces. They are being assisted by a US-backed force known as the 1920 Revolution Brigade, a Sunni militia that previously fought against the Americans but is has now turned on al-Qaeda. Members of the force have permits, supplied by the US military, identifying them as “Concerned Local Nationals” and are allowed to carry weapons.

The latest US military offensive is part of President Bush’s “surge” campaign in Iraq, where 25,000 American reinforcements have been brought in to try and restore security to Baghdad.

While the additional five American combat brigades have had some impact on the capital, commanders know that the city will never be secure as long as al-Qaeda is able to launch devastating suicide car bomb attacks on public places.

It is hoped that the Diyala operation will throw al-Qaeda in Iraq on the defensive over the coming weeks and months.

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