Security guards have killed would-be suicide bombers who apparently targeted the office of a US-linked aid group in southern Afghanistan.

The attackers approached the International Relief and Development (IRD) office in Lashkar Gah on Wednesday, but were shot dead before they could detonate their explosives, Dawood Ahmadi, a spokesman for the Helmand province government, said.

"The two suicide bombers were shot and killed by the IRD guards, luckily their explosives did not explode," Ahmadi said.

The would-be bombers were wearing vests laden with explosives.

Lashkar Gar is the closest major town to Marjah, where Nato forces have been undertaking operations to push out the Taliban.

About 15,000 US Marines, Nato and Afghan troops took part in Operation Moshtarak, but faced only sporadic resistance as many Taliban fighters apparently melted away in the face of the offensive. 

Helmand has seen increasing attacks recently by the Taliban against US and Nato forces in the country.

Washington entered the country in 2001 to remove the Taliban, whom they accused of harbouring al-Qaeda operatives, from power following the September 11 attacks of the same year.