Roadside bombs killed nearly a dozen people in Iraq yesterday, as US-led forces launched the latest phase of an offensive, killing 37 insurgents near the Syrian border, in a bid to secure the nation for December polls.

In the first roadside bomb attack, two South African private security contractors were killed by a roadside bomb near the heavily fortified Green Zone in central Baghdad, the US embassy spokesman's office said.

Three others, an American, an Iraqi, and another South African, were wounded in the attack, two of them seriously. Earlier Iraqi police had said three people were killed.

The contractors work for Dyncorp, a US-based firm that has a range of security operations in Iraq, including employing Westerners to protect convoys and reconstruction projects.

In other violence, six civilians died when a similar roadside explosive device hit two passenger coaches in Ramadi, a violent city about 100 km west of the Iraqi capital.

Ramadi is the capital of Anbar province, a vast Sunni Arab region that is a focal point of the insurgency against the Shi'ite- and Kurdish-led government and its US backers.

Sectarian tensions are overshadowing campaigning for December 15 parliamentary elections, where the once-dominant Sunni minority is expected to vote in large numbers for the first time after boycotting the first post-Saddam Hussein vote in January.

The blast near the Green Zone, which could be heard several kilometres away and sent a thick plume of black smoke into the air, was near a compound housing the Iranian embassy, but there were no reports of injuries or damage to the mission.

Police said the attack targeted several sports utility vehicles, commonly used by the military, police and security forces to transport local and foreign officials and contractors.