"Four of our staffers were killed in Baghdad yesterday," company spokesman Tim O'Brian said on Friday.
A foreign office spokesman in London said the four dead were Nepalese nationals who formerly served with Britain's famous Gurkha army regiment.
But he gave no further details about Thursday's attack.
"It's not clear yet what happened," he said.
Two loud explosions echoed from the Green Zone on Thursday and thick black smoke was seen rising above the compound.
O'Brian said he did not want to reveal the nationalities of the victims until next of kin had been notified, but he said several other employees had been wounded in the attack.
The Green Zone, a sprawling complex surrounded by blast walls, houses the US and British embassies and the interim Iraqi government and parliament.
Anti-US fighters regularly fire rockets and mortars at the compound, and have detonated several car bombs close to its entrances.