Guess Who Pays the Price for Stupidity.

Two U.S. troops have been shot to death and four more wounded by an Afghan solider who turned his gun on his allies in apparent anger over the burning of Korans at a U.S. military base in Afghanistan, an Afghan official tells CBS News.

A statement from the International Security Assistance Force - Afghanistan, the international coalition in the country, confirmed that two troops were killed in Eastern Afghanistan on Thursday by "an individual wearing an Afghan National Army uniform."

ISAF does not typically give the nationality of casualties until family members have been notified, but the CBS News source in the Afghan government said those killed and injured in the attack in the eastern Ningarhar province, along the border with Pakistan, were Americans.

The source also said the shooting appeared to be motivated by the burning of Korans at the sprawling U.S. Bagram air base, north of Kabul, but he did not provide additional details as to what led him to that conclusion.

The suspect apparently joined other protesters already demonstrating against the U.S. at an American military outpost and opened fire with an automatic weapon, according to the Afghan source.

There have been violent anti-U.S. protests for three days across Afghanistan, since the American military apologized for what it said was the accidental "improper disposal" of religious materials, including Muslim holy books, at Bagram. The U.S. is cooperating with the Afghan government to investigate the incident.

The protests Thursday at U.S. and NATO military bases around Afghanistan and in the capital city of Kabul saw renewed clashes between demonstrators and police, with security forces in Kabul reportedly opening fire and wounding several protesters. Five protesters were reportedly killed by police at protests in the north and south of the country.

For U.S. and NATO commanders in Afghanistan, the main concern is what may come after Friday prayers in 24 hours. Friday is the holy day in the Muslim week, and protests are typically much larger as thousands of Muslim men flood out of mosques and converge in cities and towns in protest.

While calls from some Afghan parliamentarians for citizens to try and attack Americans are unhelpful - especially coming from an ally - they pale in significance against the potential damage which the religious leaders could inflict if they urge similar attacks in their Friday prayer speeches.

Afghan President Hamid Karzai's office said Thursday that President Obama had sent a letter to him formally apologizing for the incident at Bagram, and top U.S. commander Gen. John Allen, who ordered the investigation, is making intense efforts to keep that probe as open as possible, but the investigation is not appeasing Afghans, who are tired of apologies.