U.S. President Barack Obama apologized to Afghan President Hamid Karzai Thursday for NATO troops' burning of Korans, which has led to violent protests.

An Afghan soldier, apparently angry over the burning of Korans at a U.S. air base, fatally shot two U.S. troops and wounded four others, Afghan officials said.

National Security Council spokesman Tommy Vietor and Karzai's office said Obama had called the burning of the Muslim sacred text at Bagram Air Base this week an "error," CNN reported.