Afghan protesters have taken to the streets for a fourth consecutive day over the burning of Korans by American troops at a US air base.Crowds in Kabul are heading towards the city centre and protests have also been reported in northern Afghanistan.On Thursday at least eight people died, including two US soldiers, as violence flared in the north and east.US President Barack Obama also apologised to the Afghan people saying, the incident was a "genuine mistake”.In a letter to President Hamid Karzai, Mr Obama expressed his "deep regret”. Mr Obama’s letter, delivered by the US ambassador to Afghanistan, assured the Afghan president that US authorities would question all those responsible.Muslims consider the Koran the literal word of God and treat each book with deep reverence.