US and Iraqi forces are carrying out a major security operation in the northern city of Kirkuk, where a 12-hour curfew was imposed.
Main routes in and out of Kirkuk were sealed
Iraqi police said that so far more than 20 suspects had been detained.
They said more then 3,500 Iraqi forces and hundreds of US soldiers were targeting al-Qaeda in Iraq militants.
Correspondents say there are rumours senior al-Qaeda activists may have made their way to Kirkuk to elude the surge in security in and around Baghdad.
The BBC's Jim Muir in the capital says the curfew was imposed from 0500 local time (0200 GMT) without warning by the authorities in Kirkuk.
Main routes in and out of the city were sealed.
Iraqi police and army units combed the city, searching homes and raiding other buildings suspected of harbouring insurgent elements.
Kirkuk's police chief said that a number of known suspects were among those detained for questioning.
Our correspondent says the city has been the subject of several major security dragnets in the past, but that has not prevented it being shaken from time to time by car bombs or suicide attacks.
Kirkuk is home to a volatile mix of Kurds, Arabs and Turkomans, all of whom dispute its historical identity.
The Kurds want to see the oil-rich province of which Kirkuk is the capital attached to their own autonomous area in the north.
Under the Iraqi constitution a referendum on the issue is supposed to take place by the end of this year but preparations have been delayed.