Four Nepalese former Gurkhas working for a UK security firm have been killed inside the Green Zone in Baghdad.
London-based Global Risk Strategies confirmed that 15 workers had also been injured in Thursday's incident in the Iraqi capital.
The firm refused to reveal the identities of those killed, saying next of kin were being notified.
The Foreign Office confirmed the men were former Gurkhas but gave no further details of the attack.
Tim O'Brien, a spokesman for Global Risk Strategies, said: "We can't confirm what this incident actually was until we go through internal investigations."
He said none of those injured in the attack were British.
It has been separately reported that a rocket attack killed four people in the Green Zone on Thursday.
The high-security Green Zone is Iraq's administrative centre, and houses the US embassy and many foreigners.
This incident has been not been confirmed as the cause of the Gurkhas' deaths.
Global Risk Strategies has around 1,500 contractors of various nationalities working in Iraq.
Earlier this month, former policeman John Barker, from Leeds in Kent, was killed in a suicide bombing, while working on a project for the company at Baghdad Airport.
Another contractor, Julian Davies, from Barry in south Wales - a former sergeant in the Territorial Army ranks of the SAS - was shot in the northern city of Mosul in June.