[Updated at 10:35 a.m. ET] Two American troops were killed in Afghanistan on Thursday near a demonstration where people were protesting the burning of Qurans by NATO troops.

The troops were killed by a person wearing an Afghan National Army uniform, a U.S. official said, asking not to be named discussing casualties.

The troops are among at least nine people who have been killed near or amid demonstrations that have erupted in Afghanistan since the burning of the Islamic religious material by NATO troops at the beginning of the week. It is not clear if the troops were killed in revenge for the burning of Qurans, but the attack occurred at a base outside of which the demonstration was taking place, a local official said.

The protest and shootings came as the Taliban called on Muslims to attack NATO military bases and convoys and kill its soldiers following the admission that NATO troops had incinerated Islamic religious material at Bagram Airfield.

Afghan officials investigating the Quran burning urged Afghans to respond wisely and avoid protests "that pave the ground for the enemies of peace," they said in a statement Thursday.

[Initial post, 8:09 a.m. ET] President Barack Obama apologized to Afghan President Hamid Karzai for the burning of Qurans by NATO troops, calling the act an inadvertent error, Karzai's office and National Security Council spokesman Tommy Vietor said Thursday.

"We will take the appropriate steps to avoid any recurrence, including holding accountable those responsible," Obama said in the letter, according to Karzai's office.

Hundreds of demonstrators gathered outside the airfield Tuesday, furious over reports of the burning.

Belief Blog: Rioting over Quran burning is un-Islamic, scholar argues

Some Qurans and other Islamic religious materials gathered for disposal from a detention facility at Bagram Airfield in Afghanistan were improperly burned, the commander of the International Security Assistance Force said Tuesday.

"This was not a decision that was made because they were religious materials," Gen. John Allen said. "It was not a decision that was made with respect to the faith of Islam. It was a mistake. It was an error. The moment we found out about it, we immediately stopped and we intervened."