"The rocket landed on the roof of the parliament right above the speaker's office," said Sheikh Sabah Saadi, an MP from the Shia Fadhila Party. "There are no casualties but there is slight damage to the office."

The elected body was not in session and few lawmakers were in the building, but after the attack employees were prevented from leaving.

The Green Zone, the seat of the Iraqi government and home of the US and British embassies, has been a regular target of mortar strikes over the past month.

Two people were killed and another eight wounded when 10 mortars slammed into the area last week, and another four Asian contractors were killed in a similar attack at the beginning of the month.

Meanwhile, gunmen opened fire on a minibus northeast of Baghdad on Monday, killing seven people before setting the vehicle on fire and incinerating the bodies, a local hospital official said.

"The minibus was carrying 11 passengers when it was attacked and then set on fire," said Mohammed Abdullah, an official in the main hospital in Baquba, capital of the violent province of Diyala.

"When the ambulances arrived at the scene they found seven bodies and four wounded people," he said, adding that one of the dead was a child and all four of the wounded were women.

A few hours later two more were killed and 15 were wounded when three mortars crashed outside a bank where people were lined up to collect their salaries, according to police Lieutenant Ahmed Ali.

In the past weeks, the mixed province of Diyala has seen some worst violence in the country with Sunni insurgents targeting the local Shia population as well as fighting pitched battles with US and Iraqi forces.

In a separate attack, a roadside bomb blast killed three Iraqi soldiers and wounded another two as they patrolled a west Baghdad suburb, according to two security sources.

Attacks against Iraqi security forces have intensified in recent weeks as the US military and the Shia-led Iraqi government have flooded the capital with soldiers and police in an effort to quell the daily bloodshed.

Earlier US forces freed five Iraqi hostages showing signs of torture in a raid in western Baghdad on Monday, the military said.

US forces discovered the hostages when they opened a padlocked room in a building they were searching in Karmah, a restive town north of Fallujah in the western Al-Anbar province, the military said in a statement.

"Inside were four men and a boy who had been kidnapped and severely beaten with chains, cables and hoses," the statement said, adding that the hostages bore the marks of torture.

"The boy stated the terrorists had hooked electrical wires to his tongue and shocked him," it said.

After being treated for their injuries and returned to their families the hostages told US forces that they believed their captors were foreign Arabs, judging by their accents.

Across Anbar, tribes and villagers who once supported the anti-US insurgency are increasingly turning against al-Qaeda in Iraq, which US forces say terrorises the populace with kidnappings and assassinations.