A rogue Afghan soldier shot dead two American soldiers and wounded four more during protests over the burning of Qurans at a U.S. base, according to a report.

CBS news reported that the Afghan National Army soldier turned his gun on U.S. soldiers in eastern Afghanistan during Thursday's demonstrations against the burning of the Quran and other Islamic relics at Bagram airbase, north of Kabul, earlier in the week.

In a bid to calm escalating protests about the burning, President Barack Obama formerly apologized to Afghan President Hamid Karzai on Thursday.

In a statement, Karzai's office said that in a letter, Obama expressed his deep regret for the reported incident, adding: The error was inadvertent. I assure you that we will take the appropriate steps to avoid any recurrence, to include holding accountable those responsible.

The killings came only hours after the Taliban urged Afghans to attack military bases and Westerners in retaliation for the Quran burnings, which U.S. authorities say were unintentional.

An earlier statement from the International Security Assistance Force -- Afghanistan, or ISAF, confirmed only that two coalition soldiers had been killed by an individual wearing Afghan National Army uniform.

ISAF do not typically give nationalities of dead and wounded soldiers until their families have been informed.

The American deaths came as it emerged a series of attacks by suspected al-Qaeda militants across Iraq on Thursday killed at least 50 people.

The attacks, which lasted for four hours, were part of a series of coordinated bombings and shootings in Baghdad and 11 other cities intended to destabilize the country.

Anger over the Bagram book burning has been building since Tuesday, when Afghan workers at the sprawling Bagram airbase saw soldiers dumping books, including Qurans, into a garbage pit and burning them.

At the time the top U.S. and NATO commander, Gen. John Allen, issued an apology and telephoned Afghan President Karzai to explain the mistake, Reuters reported.

So far, 11 people have been killed and 17 wounded as U.S. troops, Afghan police and the Afghan army struggle to contain growing protests over the Quran burnings.