BAGHDAD - Two simultaneous car bombs blasted a busy market in central Baghdad on Monday, killing at least 67 people in fresh violence that came as Iraqis waited the start of a planned US-backed offensive in the capital.

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After a relative lull in violence in the capital at the weekend, the car bombs exploded barely a second apart in a market for second-hand goods in the crowded Bab al-Sharji area, just after midday (0900 GMT).

Deputy Health Minister Hakim al-Zamely told Reuters that 142 people were wounded and the death toll of 67 could rise.

It was the bloodiest attack since at least 70 people were killed in a double bombing outside a Baghdad university six days ago. Shi'ite Prime Minister Nuri al-Maliki blamed those attacks on supporters of Saddam Hussein reacting to the execution of the ousted president. Such insurgents are mostly Sunni Arabs.

Reuters reporters heard the two blasts, which sent thick clouds of black smoke into the air.

In the aftermath of the explosions, bodies lay charred on the ground in front of market stalls that were mangled and blackened. Minibuses and other cars ferried the wounded away as firemen put out the flames.

US President George W. Bush is sending more than 21,500 fresh troops to Iraq to help the Shi'ite-led Iraqi government stop Shi'ite death squads and minority Sunni insurgents blamed for spiralling sectarian violence that threatens all-out civil war.

Most of the reinforcements, 17,500, are to be deployed in Baghdad, where US generals say previous offensives failed because there were too few boots on the ground to hold neighbourhoods that had been cleared in house-to-house sweeps -- and because the government failed to go after Shi'ite militias.

Monday's blasts came after a particularly bloody weekend for US forces in Iraq. Twenty-seven servicemen were killed in a helicopter crash, clashes with militants and roadside bombs. All but two were killed on Saturday, the third deadliest day for US troops since the war started in March 2003.

DISTRICT SEALED

Early on Monday, US-backed Iraqi forces sealed off a predominantly Sunni neighbourhood in Baghdad, but the Defence Ministry said it was not the start of a promised new offensive.

Residents of Adhamiya, in northern Baghdad, said Iraqi soldiers had set up checkpoints on roads leading into the area and were preventing motorists from passing through. They said the operation followed several nights of shooting.

An armoured force of American troops in Stryker vehicles was also seen, other witnesses said. They reported seeing residents walking out of the district on foot towards a nearby highway to catch lifts from passing cars.

The US military said on Sunday that about 3,200 fresh soldiers had arrived in Baghdad, the epicentre of the sectarian violence. Securing the capital is seen as crucial to stopping Iraq's descent into civil war.

Maliki announced a major security plan for Baghdad earlier this month, vowing to crush illegal armed groups "regardless of sect or politics".

Defence Ministry spokesman Mohammed al-Askari told a news conference that the Adhamiya operation was not the start of the crackdown: "The Baghdad Security Plan has not started yet."

US military spokesman Lieutenant-Colonel Christopher Garver said: "There are ongoing operations in Baghdad. We are continuing to clean areas and confiscate weapons."

When asked whether Monday's local clampdown was the start of the planned new sweep across the city, he said: "We are not going to witness any big announcements."