A Briton was among five G4S employees killed in a blast in Kabul, the security company has said.
A total of 10 people are confirmed to have died in the attack, which hit the firm's compound late on Wednesday.
The Taliban has said it was behind the attack.
G4S said 32 of its employees were injured and five killed, four of whom were Afghan nationals and one British.
"Our thoughts at this difficult time are with the loved ones of those who have died and been injured, and our brave team in Afghanistan who have lost colleagues and friends," managing director Charlie Burbridge said.
"We are offering support and counselling to all of those affected."
"We are committed to our security role in support of the people of Afghanistan, and we are determined that incidents such as this will not prevent the vital work that the international community conducts from continuing."
In the attack, the British firm's security compound was hit by a massive vehicle bomb, with four attackers entering the building on foot after the detonation.
Parts of nearby buildings in the industrial area of Kabul were twisted by the violence of the blast, and passers-by were hurt as debris was sent flying.
"I lost consciousness but regained it in a few minutes," Abdul Mohammad, who was caught in the blast, said.
"I felt a lot of debris hitting us. I tried to pick my nephew up but couldn't do it… I realised he was dead."
According to its website, G4S holds contracts to provide security to diplomats and other staff members working for the Foreign and Commonwealth Office in Afghanistan.
It has been targeted by insurgents before: In March, a suicide bomber detonated explosives before reaching the entrance of the compound.
Wednesday's attack came just hours after Ashraf Ghani, the Afghan president, announced that the government would be renewing negotiations with the Taliban, raising hopes of finally bringing the 17-year war in the country to an end.
The Taliban rejected the plans as a "waste of time" and considered the government "impotent".
In the years since the US-led invasion in 2001, private security firms like G4S have become embedded in Afghanistan, and the country has remained mired in protracted and bloody conflict.
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A top commander in February told Sky News that the threat of attacks on the capital is worse than ever, and in 2017 some 10,453 people were killed or injured.
G4S said the families of those hurt had been informed and that operations in Kabul were being supported by other private security companies, NATO, Afghan and British armed forces and the British embassy.