U.S. President Barack Obama has rendered his apology for the burning of Quran by his forces in Afghanistan, a statement of Afghan Presidential Palace said Thursday.“US President, Barack Obama sent President Karzai a letter to apologize for the incident in which copies of the Holy Quran were burnt in Bagram military base,” the statement added.In the official letter conveyed to the Afghan President this afternoon by Ryan Crocker, the US ambassador to Afghanistan, President Obama has written that the incident in their facility was not intentional and assured the Afghan President of full investigation, the statement stated.According to the statement, President Obama writes, “I wish to express my deep regret for the reported incident. I extend to you and the Afghan people my sincere apologies.”“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 letter concludes, according to the statement.The reported Quran burning on Tuesday in the U.S. military center at Bagram airbase 50 km north of Kabul, where hundreds of suspected Taliban and al-Qaida operatives have been held, has triggered protest demonstrations over the past three days in Afghanistan which have killed 13 people dead and injured over 50 others.