BAGHDAD (Reuters) - Baghdad’s fortified “Green Zone” came under repeated rocket or mortar attack on Sunday, and police said up to 17 people had been killed by rockets falling short outside the government and diplomatic compound.

The attacks were part of a wider increase in violence in the capital and in the northern city of Mosul, underlining warnings by U.S. military commanders that recent security gains in Iraq are both fragile and reversible.

In the past, the U.S. military has blamed such attacks on the Green Zone on rogue elements of Shi’ite cleric Moqtada al- Sadr’s Mehdi Army militia. Sadr has imposed a ceasefire on the militia, but there have been signs that it is fraying.

In Mosul, a suicide truck bomber killed 15 Iraqi soldiers and wounded 45 people, including civilians, in an attack on an Iraqi army base, the Interior Ministry said. U.S. commanders describe Mosul as al Qaeda’s last urban stronghold in Iraq.

Much of Sunday’s violence took place in Baghdad, the centre 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 U.S. and Iraqi forces has since sharply reduced levels of violence.

Gunmen in three cars opened fire on pedestrians in the 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 neighborhood, a suicide car bomber killed six people waiting in a petrol queue, police said.

ATTACKS UNUSUAL

The U.S.-protected Green Zone in central Baghdad was hit by at least three separate barrages in the morning and after night fell, suggesting that militants are becoming bolder. It is unusual for there to be so many attacks in a single day.

A U.S. embassy spokesman said the attacks wounded five people in the Green Zone.

The Green Zone was often hit by rockets and mortars at the height of sectarian violence a year ago, but attacks have become rarer as security has improved across Iraq.

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Baghdad police said 17 people were killed when mortars and Katyusha rockets, either randomly aimed or which misfired, hit three Baghdad neighborhoods during the attack on the Green Zone. Interior Ministry sources said five people were killed.

In those attacks, police said one mortar hit a house in downtown Baghdad killing a man and woman and their three sons.

U.S. embassy officials confirmed “indirect fire” attacks on the Green Zone, a term used to describe rocket or mortar fire.

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“The assessment at this time is that it caused no deaths or major casualties,” said an embassy spokeswoman.

A large plume of thick black smoke could be seen rising from one part of the Green Zone, which houses many government ministries and diplomatic missions, including the U.S. embassy. Sirens could be heard warning people to take cover.

While there was no immediate claim of responsibility, suspicion is likely to fall on Sadr’s Mehdi Army, which last week clashed with Iraqi and U.S. forces in the southern city of Kut and southern Baghdad.

Mehdi Army fighters are known to be chafing over the seven- month-old ceasefire, which they complain is being exploited by U.S. forces and police allied to a rival Shi’ite faction to target them in the oil-producing south of the country.

The U.S. 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.

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