The Afghan president on Sunday strongly condemned the killing of 16 civilians, including nine children and three women, by a U.S. soldier in the country's southern province of Kandahar, said a statement of Karzai's office.

Afghan National Army soldiers keep watch as Afghans gather around the killing site in Kandahar province, March 11, 2012. [Photo/Agencies]

"Afghan President Hamid Karzai in strongest terms condemned killing of 16 civilians by a U.S. troops in Kandahar province describing it as 'intentional killing of human'," the statement said.

Five more people were injured in the incident, which occurred in the wee hours Sunday in Zangabad village of Panjwayi district in the province some 450 km south of capital city Kabul, it added.

"The Afghan gov't had in the past strongly condemned civilian casualties that caused during the military operations by international forces that was called war on terrorism. But when a U.S. soldier intentionally kills innocent people, it could be described as an act of terror, which cannot be forgiven," Karzai said in the statement.

Previously, the Afghan Defense Ministry has condemned the killing by a soldier from the NATO-led International Security Assistance Force (ISAF) and called for the punishment of those responsible for the callous act.

Earlier on Sunday, the ISAF in a statement said that "a United States service member was detained today in connection to an incident that resulted in Afghan casualties in Kandahar province."

The security in-charge in southern region Assadullah Khalid who headed a delegation to probe the incident confirmed to Xinhua that a U.S. soldier who walked out his military base in Panjwayi district broke into three houses and opened indiscriminate fire on them.

In his statement, Karzai said that people of Afghanistan and the Afghan government asked the U.S. government to explain and clarify the incident as soon as possible.

Karzai also made telephone calls to people who sustained injuries in the incident. An Afghan teenage namely Rafiullah told the president that an American soldier broke into their house in darkness and opened fire on his family after calling them to stand on their beds.

According to Khalid, the attacks took place at about 3:00 a.m. Sunday and the attacked houses were about 500 to 600 meters from the base located in Balandi area of the district.

Meantime, in a separate statement, ISAF top commander Gen. John R. Allen, currently visiting Washington, said he was shocked and saddened by the incident.

"I offer my profound regret and deepest condolences to the victims and their families. I pledge to all the noble people of Afghanistan my commitment to a rapid and thorough investigation," Allen said in the statement.

"In the meantime, we will continue to offer medical care for those who were injured in this shooting. We will maintain custody of the U.S. service member alleged to have perpetrated this attack. And we will cooperate fully with local Afghan authorities as we ascertain all the facts," the top U.S. commander said.

The shootout incident happened in the wake of the violent nationwide protests against the U.S. troops in Afghanistan after some Koran books were found burnt at a U.S. base, raising concerns over a new wave of demonstration in the making.