A bomb has rocked the centre of the Iraqi capital, Baghdad, sending thick smoke into the air near the heavily guarded area known as the Green Zone. The attack happened near the offices of the interim Iraqi government and the US and British embassies. At least four people are reported killed by the blast, which happened as people queued to get into the zone. The explosion occurred at a place where cars queue to drop off and pick up people in the Green Zone, the huge closed-off complex that until June was the headquarters of the US-led authorities in Iraq. Those waiting to get inside the zone were office workers, cleaners and journalists. He said they had just passed a number of new Toyota pick-up trucks and were waiting to get security clearance when one of those vehicles exploded barely 50m away. He says it is too early to say it was a suicide attack, although at least four bodies were dragged from the burning wreckage. US forces quickly took control, cordoning off streets with tanks, while helicopters patrolled overhead. The blast comes after a period of relative calm in Baghdad. Our correspondent says it is unclear what the target was or whether it was a generalised attack on people doing business with the new Iraqi administration.