The NATO-led Resolute Support Mission in a statement confirmed the incident and said two Americans have sustained injuries but the RS and USA forces Commander Gen. Scott Miller has escaped the attack unhurt.
Officials confirmed three Americans were hurt - a service member, a civilian and a contractor - but the injuries are not life threatening.
An Afghan lawmaker contesting this week's parliamentary elections was among four people killed on Wednesday by a bomb planted under his office chair, officials said, in an attack claimed by Taliban militants.
Afghanistan witnessed a string of attacks ahead of the October 20 parliamentary elections.
Mr. Mattis said he had yet spoken to Gen. Miller and could not confirm the Taliban's claim of responsibility. "Initial reports also say the attacker is dead".
Wahida Shakar, spokeswoman for the provincial governor, says three Afghan civilians were also wounded.
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Another major attack past year inside the Kandahar governor's office took a heavy toll on officials, killing a deputy governor, the ambassador of the United Arab Emirates, and members of Parliament.
He was visiting the Kandahar governor's compound for a meeting with the governor and other provincial officials.
In addition to Kandahar police chief Gen. Abdul Raziq's death, the local leader of the country's main intelligence agency and a journalist were killed, Datyagiri said. "Responding to the shooting, Pentagon spokesman Lieutenant Colonel Kone Faulkner told Reuters that the attack will not change the United States resolve in its South Asia strategy if anything it made them more resolute".
Raziq took over as Kandahar's police chief after his predecessor, Khan Mahammad Mojayed, was killed in a suicide attack in 2011.
Security officials in Afghanistan had warned of attacks ahead of the election but Razeq's death triggered deep shock that officials fear may keep voters away, particularly after Taliban warnings not to participate in what the militants consider a foreign-imposed ballot. North Atlantic Treaty Organisation officials say Miller escaped unharmed. "Strengthening of democracy is key to long-term peace and security in Afghanistan", the statement said. Zalmay Wesa, Kandahar's provincial governor, and Abdul Mohmin, local commander of the National Directorate of Security, were also killed.
Smith said he had brought a measure of calm to a city that was experiencing daily explosions when he took command of the province's police force in 2011.
"We had served together for many years". At the same time, he earned a reputation as a ferocious opponent of the Taliban and gained the respect of successive American and North Atlantic Treaty Organisation military officials in Afghanistan. It came just days ahead of the Afghan elections.