U.S. President Barack Obama on Sunday called Afghan President Hamid Karzai to express his "shock and sadness" at the reported killing and wounding of Afghan civilians.

"President Obama extended his condolences to the people of Afghanistan, and made clear his administration's commitment to establish the facts as quickly as possible and to hold fully accountable anyone responsible," the White House said in a statement.

Obama also "reaffirmed our deep respect for the Afghan people and the bonds between our two countries," the White House said.

In a statement released earlier the day, the president described the incident as "tragic and shocking," saying it "does not represent the exceptional character of our military and the respect that the United States has for the people of Afghanistan."

The incident, in which at least 16 civilians, including women and children, were killed by a U.S. soldier who walked out from his military base on Sunday morning and opened fire on villagers in the southern province of Kandahar, came as relations between Washington and Kabul have been strained over the burning of copies of Quran by U.S. troops last month.

Karzai condemned the killing as an "unforgivable action," demanding an "explanation" from Washington.

The two countries are negotiating a U.S.-Afghan strategic partnership document that will guide bilateral ties beyond 2014, when American and NATO forces are scheduled to hand over security responsibility to the Afghan forces and end combat mission.