At least another eight people have been killed, including two NATO soldiers, in violence across Afghanistan, after the burning of the Koran at a U.S. base.

Many others were injured in the protests, while armed men also attacked at least two military installations.

U.S. President Barack Obama has sent his Afghan counterpart Hamid Karzai a letter of apology for the burnings.

Crowds shouting "death to Obama" have been throwing stones and setting fire to the U.S. flag.

Obama's letter, delivered by the U.S. ambassador to Afghanistan, assured the Afghan president that US authorities would question all those responsible. "I convey my deep sympathies and ask you and the people to accept my deep apologies," the letter said.

Meanwhile the Taliban has called on Afghans to kill and beat all Westerners in revenge for "insulting" the Koran.

In a statement a Taliban spokesman said Afghans should "not stop at protesting" but instead target military bases and personnel to "teach them a lesson that they will never again dare to insult the Holy Koran".