Baghdad's heavily fortified "Green Zone" came under heavy rocket or mortar attack on Sunday, and police said up to eight people had been killed by rockets falling short outside the government and diplomatic compound.

The missile attack on the Green Zone came amid an increase in violence in the capital and in the northern city of Mosul, underlining warnings by US military commanders that recent security gains in Iraq are both fragile and reversible.

In Mosul, a suicide truck bomber killed 13 Iraqi soldiers and wounded 42, including civilians, in an attack on an Iraqi army base, the Interior Ministry said. US commanders describe Mosul as al Qaeda's last urban stronghold in Iraq.

Much of Sunday's violence took place in Baghdad, the epicentre of sectarian bloodletting between Iraq's majority Shi'ites and minority Sunni Muslims in 2006 and 2007 in which tens of thousands died. A security crackdown by US and Iraqi forces has since sharply reduced levels of violence.

Gunmen in three cars opened fire on pedestrians in the capital's religiously mixed southern district of Zaafariniya, killing at least seven and wounding 16, police said.

Blood and bullet casings littered the street in front of a clinic, market and a housing compound.

"I heard that my brother was killed. I just want to know how the terrorists got through all the checkpoints to reach here," said Zaafaraniya resident Abu Mohammed.

In northwestern Baghdad's Shula district, a predominantly Shi'ite neighbourhood, a suicide car bomber killed six people waiting in a petrol queue, police said.

The US-protected Green Zone in central Baghdad area was often hit at the height of sectarian violence a year ago, but attacks have become rarer with improved security across Iraq.

Baghdad police said nine people were killed when Katyusha rockets, either randomly aimed or which misfired, hit two Baghdad neighbourhoods during the attack on the Green Zone. Interior Ministry sources said five were killed.

The US military meanwhile said it killed 12 insurgents in a raid on a house east of Baquba in volatile Diyala province.

"Six of the terrorists killed had shaved their bodies, which is consistent with final preparation for suicide operations," spokesman Major Winfield Danielson said.

Mosul and Baquba are the capitals of two of four northern provinces where offensives were launched this year against Sunni Islamist al Qaeda fighters who regrouped there after being driven out of strongholds around Baghdad and western Anbar.

While there was no immediate indication of who was responsible for the Green Zone attacks, the US military has blamed past missile strikes on rogue elements of anti-US Shi'ite cleric Moqtada al-Sadr's Mehdi Army militia.

Sadr last month renewed a seven-month-old ceasefire for his militia, which the US military has credited for contributing to sharp falls in violence across Iraq.

However, there are fears the ceasefire may be unravelling after Mehdi Army fighters clashed with Iraqi and US forces in the southern city of Kut and southern Baghdad last week.

The Iraq war last week moved into its sixth year, US President George W Bush marking the anniversary of the US-led invasion to topple Saddam Hussein with an upbeat speech in which he said the United States was on track to victory. - Reuters