WASHINGTON -?The US government on Sunday offered condolences to families of the Afghan civilians who were killed by a US soldier, saying that Washington was saddened by the "violent act."

"The United States extends deepest condolences to the families of today's tragic shooting and we're saddened by this violent act against our Afghan friends," a State Department spokesperson said in a statement.

"Our thoughts and prayers are with the families," the statement added.

According to reports, at least 16 civilians, including women and children, were killed by a US soldier who walked out from his military base on Sunday morning and opened fire on villagers in the Afghan southern province of Kandahar.

Amid growing anger on the side of Afghanistan, its president Hamid Karzai on Sunday condemned the killing as an "unforgivable action," demanding an "explanation" from Washington.

The incident will likely worsen the already tense relationship between the United States and Afghanistan. Just last month, the Quran burning by US troops in Afghanistan had triggered deadly protests and violence in the country.