Bombers and gunmen have launched an apparently co-ordinated string of attacks against Iraqi government forces, killing at least 46 people a day after the number of US troops fell below 50,000 for the first time since the start of the war.

The violence highlighted persistent fears about the ability of Iraqi troops to protect their own country as the American military starts to leave.

There were no claims of responsibility for the spate of attacks. But their scale and reach, from one end of the country to the other, underscored insurgent efforts to prove their might against security forces and political leaders who are charged with the day-to-day running and stability of Iraq.

The deadliest attack came in Kut, 100 miles south-east of Baghdad, where a suicide bomber blew up a car inside a security barrier between a police station and the provincial government's headquarters. Police and hospital officials said 16 people were killed, all but one of them policemen. An estimated 90 people were wounded.

Government employee Yahya al-Shimari, 40, was going to work when the blast hit.

"I rushed to the scene to help evacuate the people, and saw body parts and hands scattered on the ground and dead bodies of policemen," al-Shimari said. "I also saw a traffic policeman lying dead on the ground. There were about 15 cars that were burnt."

An eerily similar attack came in a north Baghdad neighbourhood, where a suicide bomber detonated a car bomb in a parking lot behind a police station.

Fifteen people were killed in that attack, including six policemen. Police and hospital officials said another 58 were wounded in the explosion that left a crater three metres wide and trapped people beneath the rubble of collapsed houses.

Four others, including an Iraqi soldier and a police officer, were killed in small bursts of violence in Baghdad.

A senior Iraqi intelligence official raised the possibility that some of the attackers had inside help. The official said the Baghdad suicide bombing bore the hallmarks of al-Qaida, but said unnamed political factions helped co-ordinate some of the other attacks.