A US defense contractor was killed in a blast in Bulgaria last week whil working for to train and equip Syrian rebels fighting against Islamic State, US officials said Wednesday.

The American died when a rocket-propelled grenade misfired at a military range in Anevo in central Bulgaria on Saturday.

Two other US defense contractors were also badly injured in the explosion.

"The defense contractors... are employees of the company Purple Shovel, which has been awarded a contract by Special Operations Command, at the request of U.S. Central Command, to support the Combined Joint Interagency Task Force-Syria," the US embassy in Sofia said in a statement.

It specified that CJIATF-S is the organization tasked to administer the U.S. Coalition Syria Train and Equip program aimed at training moderate Syrian opposition fighters to battle Islamic State.

"The three contractors were conducting familiarization training for other company employees," it added.

Contacted by AFP, a spokesman for Purple Shovel declined to comment on the incident.

The two US contractors who survived the blast were taken to a US military hospital in Germany.