Taliban militants claimed responsibility for the bombing.
Zabihullah Majahid, who claims to speak for the Taliban outfit, claimed that first a militant blew up his explosive-laden car next to the company, opening the way for others to enter the company's compound and begin targeting anyone inside.
Earlier reports put the number of casualties at one dead and 11 injured.
G4S was yet to make any comment about the situation following the attack on its branch.
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