A British security firm said Friday that four of its employees were killed and 15 others injured in an attack in Baghdad's Green Zone (search).

Tim J. O'Brien, spokesman for the London-based Global Risk Strategies (search), said the employees were killed Thursday, but declined to say what type of attack had occurred.

"There was an incident yesterday. We lost four people and had 12 to 15 who were injured," he said. "We can't confirm what this incident actually was until we go through internal investigations."

Multiple explosions were heard Thursday and black smoke was seen rising from the fortified Green Zone, which houses the U.S. and Iraqi leadership. The protected area lies on the western bank of the Tigris River (search).

O'Brien declined to give out identities of the victims, but said that none of those killed were American.

Global Risk Strategies is a London-based firm that provides security in countries including Iraq and Afghanistan.