The explosion outside the offices of the Patriotic Union of Kurdistan (PUK) killed at least nine people and wounded more than 36 on Tuesday morning, police said.

Rescue workers believe that more bodies are buried in the rubble from the blast.

Police in Mosul said the bomber had detonated a dump truck packed with explosives outside the PUK office in the eastern part of the multicultural city 390km north of Baghdad.

Crackdown in Baghdad

In Baghdad, US and Iraqi forces sealed off the mainly Sunni district of Dura as part of a major offensive to curb violence, the US military said on Tuesday.

Dozens of concrete barriers have been put up around the homes of residents in the southern district since Friday "in an attempt to keep terrorists out of Dura", the US military said.

"All vehicles leaving or entering the neighbourhood are stopped at designated checkpoints manned by Iraqi police looking for known terrorists, bomb-making materials and illegal weapons," the military said.

The new security method was implemented as part of Operation Together Forward, the ongoing security plan launched by the Iraqi and US armed forces in mid-June.

Karbala unrest

Iraqi police have also imposed an indefinite curfew on the Shia holy city of Karbala after clashing with Shia militias on Tuesday.

Rival Shia groups jostle for influence in Karbala

Police loudspeakers announced a curfew for both vehicles and pedestrians from 11am (0700 GMT).

Clashes had broken out earlier between men loyal to Karbala's Ayatollah Mahmoud al-Hasani and police in the city.

As many as 40 of al-Hasani's militia members were arrested, a spokesman said.

"This morning, a group of soldiers and police came and arrested 30-40 followers of the ayatollah with no justification," said Dhia al-Musawi.

"This is besides detaining a number of others inside the nearby Hussainia" mosque, he said, alleging that the arrests were backed by the central government.

Elsewhere, attackers killed three bakers supplying bread for the Iraqi army and wounded a passerby in the town of Muqdadiya, 90km north east of Baghdad, police said.

On Monday night, attackers shot dead a former Iraqi army officer on a highway near Dujail, 90km north of Baghdad.

Two roadside bombs targeting police commandos exploded near the spiral minaret of Samarra, north of Baghdad, wounding three police commandos and destroying one vehicle on Tuesday.

Two policemen were also killed in Baquba, a town north of Baghdad, on Tuesday.