At least three suicide bombings struck a police headquarters in the restive central Iraqi city of Baquba on Sunday, killing three policemen, security and medical officials said.

The blasts went off at about 8:00 pm (1700 GMT) and were followed by clashes in the surrounding area of Mafreq, with officials warning the casualty toll could rise as medical teams did not have full access to the sites.

An initial car bomb was detonated by a suicide bomber outside the headquarters of a police battalion in Mafreq, and in the ensuing chaos, another suicide bomber blew himself up.

A third bomber subsequently managed to enter the battalion compound and blew himself up at the entrance to the main building.

Officials said security forces were chasing a fourth bomber as well.

Clashes also broke out in three other neighbourhoods of Baquba, which lies 60 kilometres (35 miles) north of the capital in one of the country's least stable provinces.

Elsewhere in Iraq on Sunday, six people were killed, the latest in a months-long surge in violence that the authorities have largely failed to stem despite wide-ranging operations and tightened security measures.

In northern Nineveh province, a suicide bomber drove through a police checkpoint and detonated a car rigged with explosives, killing two policemen and wounding three others and a civilian who was nearby, security and medical officials said.

The bombing struck the Shiite Turkmen city of Tal Afar.

Also in Nineveh, gunmen opened fire on an army checkpoint in the provincial capital Mosul, killing two soldiers. And an election commission worker was gunned down elsewhere in Mosul.

In south Baghdad, a roadside bomb targeting a police patrol killed a major and wounded three other policemen, officials said.

Violence so far this year has left more than 5,450 dead, the country's worst violence since 2008, and Prime Minister Nuri al-Maliki has appealed to the US for greater cooperation in the fight against militancy.

In addition to major security problems, the Iraqi government has failed to provide adequate basic services such as electricity and clean water, and corruption is widespread.

Political squabbling has paralysed the government, while parliament has passed almost no major legislation in years.