By Caroline Hawley
BBC News, Baghdad
The US military has been carrying out raids in the western Iraqi province of Anbar in a crackdown on insurgents.
US marines are involved in the offensive in al-Anbar
The focus of the operation is the town of Ramadi, the provincial capital, which has been a rebel stronghold for many months.
Night-time curfews have been imposed and a cordon of checkpoints surrounds the town, which is home to 400,000.
The offensive has been dubbed Operation River Blitz and is targeting rebels in towns along the Euphrates river.
Iraqi security forces are also taking part in the operation.
They say a prominent tribal leader and more than 10 of his relatives are among those detained.
The operation comes three months after the controversial American assault on Falluja, just east of Ramadi.
Officials acknowledged at the time that some militants had escaped from Falluja to fight on elsewhere.
The US military says Operation River Blitz is designed to defeat insurgents in Anbar province.
More Americans have been killed in Anbar than anywhere else in the country.
Only 2% of the province's population voted in last month's elections.
Shia leaders who won the vote are now trying to undermine support for the insurgency by drawing Sunnis into the political fold.
They will be hoping the military operation will not further alienate Sunnis at such a sensitive time.