Deadly anti-American protests erupted in Afghanistan for a fourth day as anger over the burning of Qurans at a NATO facility spread throughout the country.

Afghan officials say at least two people were killed during Friday's demonstrations, bringing the death toll in days of unrest to at least 15. Two American soldiers were among those killed on Thursday.

Thousands of protesters took to the streets in the capital Kabul and the eastern provinces of Nangarhar and Ghazni, following Friday prayers. Many shouted “death to America.”

The demonstrations continued despite calls for restraint from NATO and Afghan officials.

On Friday, the commander of U.S. and NATO forces in Afghanistan, General John Allen, said the joint coalition and Afghan investigation into the “mishandling of religious materials” at Bagram Air Base continues, with witnesses to Sunday's incident being interviewed.

General Allen issued a statement saying “working together with the Afghan leadership is the only way for us to correct this major error and ensure that it never happens again.”

U.S. President Barack Obama has sent a written apology to Afghan President Hamid Karzai about the desecration of Qurans at Bagram.

The U.S. Embassy in Kabul has urged Americans to avoid any unnecessary movement within the South Asian nation.

On Thursday, the Taliban issued a statement calling on Afghans to launch attacks on foreign targets in retaliation for the burning of the Muslim holy book.

Germany on Friday withdrew its troops from a base in northern Afghanistan because of the continuing demonstrations.

Around 50 German soldiers were set to leave the outpost in the Taloqan area of Takhar province by the end of March after security control was transferred to local forces.

But a German military spokesman said the troops left the base Friday and were transferred to Kunduz province after demonstrations in Taloqan.

On Wednesday, President Karzai appealed for calm, saying citizens have the right to protest, but should not resort to violence.

The Afghan delegations assigned to probe the incident have also appealed to the Afghan people to avoid resorting to protests.