President Barack Obama sent a letter to Afghan President Hamid Karzai on Thursday apologizing for the burning of copies of the Koran at U.S. military base in Afghanistan, the White House said.

“In the letter, delivered by Ambassador (Ryan) Crocker this afternoon in Kabul, the president also expressed our regret and apologies over the incident in which religious materials were unintentionally mishandled at Bagram Airbase,” White House spokesman Tommy Vietor said.

The burning of Korans at the Bagram airbase near Kabul has sparked fierce protests against U.S. and NATO forces in Afghanistan.

Read more: Obama apologizes to Karzai over Koran burning