(Adds U.S. military spokesman on Shaab shooting)

By Aseel Kami and Paul Tait

BAGHDAD, Nov 27 (Reuters) - Nine people were killed when a suicide bomber posing as a shepherd attacked police north of Baghdad on Tuesday and U.S. soldiers killed at least six civilians in a spate of shootings, security officials said.

Police at the Diyala province headquarters in Baquba, 65 km (40 miles) north of Baghdad, said they were taken by surprise when the suicide bomber herded several sheep towards a checkpoint before detonating a belt packed with explosives.

"There was nothing suspicious about him because it's an open, agricultural area and it's normal for shepherds to be around here," Diyala police lieutenant Ali Jassim told Reuters.

Jassim said the blast killed four police and two civilian men. Three women were killed when random gunfire broke out in the chaos after the explosion. U.S. commanders identified ethnically and religiously mixed Diyala as one of the most dangerous parts of Iraq after Sunni Islamist al Qaeda fighters were driven out of their former stronghold in western Anbar province into other areas.

Attacks across Iraq have fallen by 55 percent since a "surge" of 30,000 extra troops became fully deployed in mid-June, part of a security crackdown aimed at averting civil war between majority Shi'ite Muslims and minority Sunni Arabs.

The improvement in security has been attributed partly to the growing use of neighbourhood police units, organised by mainly Sunni Arab tribal leaders and based on a model pioneered in Anbar.

The fall in violence has permitted a gradual reduction in U.S. troop numbers and about 20,000 will have left by July 2008.

Overall troop levels in Iraq will fall by about 5,000 when a combat brigade pulls out of Diyala next month, though other U.S. troops from the 162,000-strong force now in Iraq will be redeployed to the province.

NEW TACTICS

U.S. commanders say the number of attacks remains too high and that al Qaeda fighters and Shi'ite militias will seek new ways to attack military and civilian targets.

Iraqis have often been angered by what they describe as the heavy-handed use of force by U.S. troops since the 2003 invasion to topple Saddam Hussein, especially in air strikes and by troops travelling in convoys of "Humvee" vehicles.

On Tuesday, two civilians were killed and four wounded when U.S. soldiers opened fire on a minibus after its driver ignored instructions, including a warning shot, to stop at a checkpoint in Baghdad, the U.S. military said in a statement.

One police source said the bus was carrying finance ministry employees through Baghdad's northern Shaab district. Another police source put the death toll at four, three of them women. Witnesses reported seeing at least one body lying in the street.

U.S. military spokesman Rear Admiral Gregory Smith said two or three people in the bus were possibly killed by ricochets.

U.S. soldiers also opened fire on a vehicle which approached a checkpoint at high speed in Baiji, 180 km (110 miles) north of Baghdad, on Monday, killing two civilian men inside, the U.S. military said. A child found wounded in the vehicle died later.

"Multi-National Force-Iraq deeply regrets when any Iraqi civilian is injured or killed, and our condolences go out to their families," a military spokesman said.

The shooting took place after two suspected insurgents were killed in raids targeting al Qaeda around Baiji.

In Ramadi west of Baghdad, U.S. forces shot and killed the civilian driver of a vehicle who failed to stop at a checkpoint on Sunday, the military said. (Additional reporting by Waleed Ibrahim and Alaa Shahine in Baghdad; Editing by Tim Pearce)

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