"The bus was running along a busy road in Pul-e-Charkhi area at around 6.30 am. A terrorist rammed his explosive-laden car into the bus, causing a heavy explosion," witness Farhad Mohammadi told Xinhua news agency.

"Most of the victims were foreign security guards onboard the bus. Initial information showed they were Nepalese," a security official said.

Twenty-five others were injured in the attack, the official said. The Taliban militant group has claimed responsibility for the attack.

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