The attack targeted a convoy carrying Afghan employees of the firm in Girishk district of the province, where 26 people, many of them civilians, were killed in twin suicide attacks on Monday, Helmand's police chief Mohammad Hussein Andiwal told reporters.
Abdul Manaf Khan, district chief of Girishk, told Reuters the bomber rammed a vehicle into the convoy and that several vehicles and a fuel tanker blew up.
Violence has escalated in the past 19 months in Afghanistan, the bloodiest period since US-led troops overthrew the Taliban in 2001.
Inspired by insurgents in Iraq, Taliban guerrillas now largely rely on suicide raids and roadside bomb attacks against the Afghan government and Western troops.
Militants carried out 103 suicide attacks between January and the end of August this year, compared to 123 attacks during the whole of 2006, the United Nations said in a new report on suicide bombings in Afghanistan published on Sunday. Bomber motivation ranged from anger at perceived foreign military occupation to government corruption and family dishonour and humiliation.
REUTERS SBC BST1705