Kabul, Afghanistan ? Afghans enraged by the alleged desecration of Islam’s holy book at a U.S. prison staged a third day of violent protests Thursday, burning an American flag in the capital and ransacking relief group offices to the south as demonstrations spread to neighboring Pakistan.

Secretary of State Condoleezza Rice promised “appropriate action” would be taken if the allegations are proven true, while key U.S. ally Saudi Arabia urged that any offenders be disciplined quickly.

Three more demonstrators were shot and killed in clashes with police, officials said, bringing the death toll to at least seven in the biggest anti-American protests in Afghanistan since the fall of the Taliban in 2001 — and presenting a fresh challenge to efforts to stabilize the country.

While most of the protesters were students, officials suggested that elements opposed to Afghanistan’s U.S.-backed government were stirring the violence, which also has targeted American troops and the United Nations.

The demonstrations could complicate President Hamid Karzai’s plans to ask for military aid on a trip to Washington this month, a prospect that has stoked a previously muted debate on how long U.S. troops should stay to secure the country, still riven by a Taliban-led rebellion. That debate may play out in parliamentary elections this year.

The Afghan leader, on a trip to Europe, has played down the violence as the growing pains of Afghan democracy.

The trigger of the unrest was a brief report in the May 9 edition of Newsweek magazine that interrogators at the U.S. prison on Guantanamo Bay, Cuba, placed Qurans in washrooms to unsettle suspects, and in one case “flushed a holy book down the toilet.”

Afghan students toss a burning paper with a drawing of President Bush next to the words Death

Desecration of the Quran is punishable by death in both Afghanistan and Pakistan, but diplomats and officials have been taken aback by the intense reaction — further inflamed by bloodshed in a police crackdown on anti-U.S. protesters Wednesday in the eastern Afghan city of Jalalabad that left four dead and more than 70 wounded.

It was unclear why demonstrations broke out this week and not after previous media reports.