KABUL -- President Barack Obama apologized to Afghans yesterday for the burning of Qurans at a U.S. military base, trying to assuage rising anti-American sentiment as an Afghan soldier fatally shot two American troops during another day of angry protests.

The U.S.-led military coalition has said the Muslim holy books were sent by mistake to a garbage burn pit at Bagram Air Field and the case is under investigation. The explanation and multiple apologies from U.S. officials have not calmed outrage over the incident.

Thousands of protesters, some shouting "Long live Islam!" and "Death to America!" staged demonstrations across Afghanistan for a third day.

The deadliest was held outside an American base in the Khogyani district of Nangarhar province in eastern Afghanistan. Two protesters were killed by Afghan police and an Afghan soldier turned his gun on American troops, killing two.

Protesters climbed the walls of a U.S. base in the east, threw stones in and adorned an outside wall with the Taliban's trademark white flag. At other sites, demonstrators burned tires or American flags. Afghan police and international troops fired guns in the air to disperse the crowds.

The protests sparked clashes with Afghan security forces that left at least five demonstrators dead. A Norwegian soldier was wounded by a hand grenade hurled into a coalition compound.

The civil unrest comes at a time when Afghan President Hamid Karzai is trying to negotiate a long-term partnership agreement with the United States to govern the activities of U.S. forces in Afghanistan after 2014, when most foreign combat troops will have left or taken on support roles.

White House press secretary Jay Carney told reporters aboard Air Force One that Obama's apology to Karzai was "appropriate, given the sensitivity" of the issue. He said the apology was part of a three-page letter to the Afghan leader. Presidential apologies are rare, but he noted that former White House press secretary Dana Perino apologized on behalf of President George W. Bush in 2008 after a U.S. serviceman shot a Quran.

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