The Iraqi city of Kirkuk has been placed under a total curfew as Iraqi troops backed by US-led coalition forces search for insurgents.
The curfew has been imposed indefinitely
Police said at least 150 people had been detained and arms and ammunition seized in the northern city.
The major operation began as a suicide car bomb hit an army checkpoint in the northern city of Tal Afar.
Four soldiers and 10 civilians died in the city held up as a model by US President George W Bush in March.
Tal Afar, to the west of Mosul, was supposed to be a showcase for US-led efforts to pacify Iraq, the BBC's Jim Muir reports from Baghdad.
Earlier this year Mr Bush spoke at length about the city, which he said had been effectively liberated from al-Qaeda control.
Correspondents say a recent upsurge of bombings in Kirkuk prompted the crackdown to try to curb the violence.
Gun and bomb attacks are a part of everyday life in Iraq
A total curfew was imposed late on Friday until further notice.
Thousands of Iraqi army and police force personnel backed by US-led coalition troops have been hunting for insurgents, while helicopters hover overhead.
Iraqi police sources say a trench 15km (eight miles) long and two metres deep has been dug around part of the city in a bid to control access.
Our correspondent says tensions have been rising in Kirkuk, which is home to Kurds, Arabs and Turkmens who claim ownership of the city and the oil-rich lands around it.
The renewed violence came as it appeared that the US might be considering a major change in policy on Iraq.
Reports of a shift came after a visit to Iraq by a senior Republican senator.
Senator John Warner, chairman of the Senate armed services committee, offered a bleak assessment of the situation and talked of the need for new options to be considered when he returned from a recent visit to Iraq.
He suggested that events there were drifting and that the time was coming for bold decisions to be made.
Correspondents say the comments are anything but routine.
While he did not specify what the decisions might be it is being suggested that he might have been preparing the ground for the White House to give up waiting for Iraqi Prime Minister Prime Minister Nouri Maliki to establish order.
That is a decision which could lead to a timetable for the return of US troops or an acceptance that a single Iraqi nation is no longer viable, the BBC's Justin Webb reports.