(Adds another Baghdad bombing, Basra shooting, US military spokesman)

By Paul Tait

BAGHDAD, April 15 (Reuters) - Up to 26 people were killed and 70 wounded in bomb attacks in three mainly Shi'ite districts of Baghdad on Sunday, police said, and two British military personnel died when two helicopters crashed north of the city.

The Britons died and another five were injured when the Puma transport helicopters crashed near a large U.S. air base in Taji, 20 km (9 miles) from Baghdad. It appeared the helicopters may have collided in mid-air, the U.S. military said.

Two car bombs earlier on Sunday killed 15 people and wounded 50 more in the al-Shurta al-Rabeia neighbourhood in southwest Baghdad. The first was detonated in a market, followed seconds later by another at a nearby intersection, police said.

They said mortar rounds also landed in the area in an apparently coordinated attack.

Twisted metal littered the market, television footage showed. Several cars were damaged in the explosion.

In the Kadhimiya district in the northwest of the capital, a police source said a suicide bomber wearing a belt packed with explosives killed six people and wounded 11 in a small bus. Another police source put the death toll at three.

In Karrada in central Baghdad, a car bomb aimed at a police patrol killed five people and wounded another 10 in a blast that rattled windows hundreds of metres away, police said.

A two-month-old, U.S.-backed security crackdown in Baghdad seen as the last-ditch attempt to avoid Iraq from sliding into all-out civil war has reduced the number of targeted killings. But U.S. and Iraqi commanders still find car and suicide bombers hard to stop.

The U.S. military said the helicopter crash near Taji did not appear to be the result of an insurgent attack.

"We have seen a rise in the number of car bomb attacks. We have been very diligent in taking down these people who build these car bombs. We are taking one cell at a time," said U.S. military spokesman Rear Admiral Mark Fox.

BRITISH HELICOPTER CRASH

The helicopters' crash brought the British death toll in Iraq since the U.S.-led invasion in 2003 to 142. Eight have been killed this month alone.

"Sadly, two personnel have died and one is very seriously injured. All of these were UK personnel," British Defence Secretary Des Browne said in a statement issued in London.

Puma helicopters normally have a three-person crew and can carry up to 16 troops.

In Mosul, 390 km north of Baghdad, four Iraqi soldiers were killed when two oil trucks driven by suicide bombers exploded outside an Iraqi military base, police said.

Health officials said another three people had died after Saturday's suicide car bomb blast at a crowded bus station in Kerbala, a holy city that is a main pilgrimage destination for Shi'ites, taking the death toll to 43. Another 200 were wounded.

Sectarian tensions between majority Shi'ites and once-dominant Sunni Arabs are high after the bombing of a Shi'ite shrine in Samarra in February 2006 unleashed a wave of violence. Tens of thousands have been killed since then.

In the volatile southern city of Basra, British forces backing up a police raid in the southwestern Hayaniya district, a Shi'ite militia stronghold, shot five gunmen who had opened fire on them overnight, the British military said.

British officials could not confirm if they had been killed or wounded.

British forces are pursuing a more aggressive policy towards Shi'ite militias as they prepare to hand over security control in Basra to Iraqi forces later this year. Stabilising the port city is crucial because it is the hub for Iraq's main oilfields.

British forces killed eight militiamen laying roadside bombs on the western outskirts of Basra on Friday night. More than 20 gunmen were hit in another operation in the city last Tuesday.

(Additional reporting by Yara Bayoumy and Aseel Kami)

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