The US embassy in Sofia has said the three US contractors hit by last week’s explosion at a firing range in Bulgaria are employees of a firm hired to work on programme to support moderate opposition forces in Syria, Reuters reported.

One US contractor was killed and two more were wounded when a rocket-propelled grenade malfunctioned at a firing range near the village of Anevo on Saturday. Two Bulgarians were also injured in the blast.

Reuters quoted the US embassy in Sofia as saying in a statement on Wednesday the contractors were working for Purple Shovel, a company hired by the US military's Special Operations Command to support the Combined Joint Interagency Task Force-Syria, the organization tasked with administering the US government-led programme to train and equip moderate opposition rebels in Syria.

"The three contractors were conducting familiarization training for other company employees at the time of the incident, " according to the statement.

The statement didn’t say what the training involved but according to a US military source, who spoke to Reuters on Tuesday, the contractors were preparing to obtain arms for the US-led effort to train moderate opposition forces to fight Islamic State militants in Syria.

US-based Purple Shovel says on its website that it “specializes in exceptional international logistics, bridging commercial-military relations, supporting worldwide stability operations, strategic procurement, supply chain optimization, community and commercial economic growth, and ensuring humanitarian assistance.”