Protests in Afghanistan flare for a second day over the burning of copies of the Koran at NATO's main base in the country.

Hundreds of demonstrators gather here in the eastern city of Jalalabad, setting a U.S. flag on fire and shouting anti-American and anti-NATO slogans.

(SOUNDBITE) (Pashto) ABDULLAH ABDULZAI, ANTI U.S. PROTESTER SAYING:

"I urge all the Afghan people to defend the Koran. Even if it costs their lives they must defend the Koran. Long live Islam, long live the Koran, death to America."

Four people have been shot dead and dozens have been wounded in protests across the country.

The American Embassy said its staff are in "lockdown", as fury mounts over what is quickly becoming a public relations disaster for the U.S.-led NATO forces.

An American commander issued an apology on Tuesday after Afghan labourers found charred copies of Islam's holy book while collecting garbage at Bagram Airbase.

Winning the hearts and mind of Afghans is critical to efforts to defeat the Taliban, but this incident, like similar ones in the past, is causing deep resentment among Afghans towards the tens of thousands of foreign troops stationed in the country.

Travis Brecher, Reuters