KABUL: President Barack Obama apologized Thursday for the burning of copies of the Muslim holy book at a US military base this week, as violent protests raging nationwide led a man dressed in an Afghan army uniform to kill two US troops.

The Afghans' furious response to the Quran burning, three days of riots in several cities nationwide, reflected the anger at what they perceive as foreign forces disrespect for Afghan laws and culture.

In a letter sent to Afghan President Hamid Karzai, Obama expressed his administration's ''regret and apologies over the incident in which religious materials were unintentionally mishandled,'' White House national security council spokesman Tommy Vietor said.

He added that the letter was delivered by Ryan Crocker, the US ambassador to Afghanistan, Thursday afternoon.

Karzai's office said Obama called the Quran burnings ''inadvertent,'' adding that the US ''will take the appropriate steps to avoid any recurrence, to include holding accountable those responsible.''

US apologies for the desecration, and an appeal from Karzai for calm, have failed to temper the anger of Afghans, who staged rallies in seven provinces Thursday, sparking clashes with Afghan police and security forces that left at least five demonstrators dead. Seven protesters were killed in clashes on Wednesday.