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 quickly disciplined.

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 has also 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.

Downplaying violence

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."

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 enflamed by bloodshed in a police crackdown on anti-U.S. protesters in the eastern Afghan city of Jalalabad on Wednesday that left four dead and over 70 wounded.

It was unclear why demonstrations broke out this week and not after previous media reports. In July 2004, for example, the Arab station Al-Jazeera ran an interview with a former Guantanamo detainee who claimed he saw a U.S. soldier stomp on the Quran and that another American soldier in the southern city of Kandahar threw a holy book into the toilet.

Pakistan protested to the U.S. government last weekend about the alleged abuse cited in the Newsweek report, giving the article wider play in the region's media than the Al-Jazeera interview may have received last year.

At the Pentagon, Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said Thursday that U.S. commanders in Afghanistan believe that local political factions -- and not the reports about the alleged desecration -- are driving the violence in Jalalabad.

Myers said the military is investigating the allegation but so far has not been able to confirm it. In one case, an inmate, in an act of defiance, ripped pages out of his Quran and stuffed them into the toilet in an attempt to back up the plumbing, Myers said, citing logs from the prison.

In the first public comment by a Mideast nation, Saudi Arabia expressed deep indignation and asked Washington to quickly investigate and discipline any perpetrators.

Saudi Arabia "appeals to the concerned American authorities to carry out a quick investigation in the matter and stresses that in case the reports were true, deterrent measures should be taken against those perpetrators to prevent its recurrence and to protect the sentiments of Muslims all over the world," the Foreign Ministry said in a report carried by the official Saudi Press Agency.

U.S. officials tried again Thursday to calm tempers, promising a thorough probe and insisting all inmates at Guantanamo, many of them Pakistanis and Afghans captured after the Sept. 11 attacks, are given Qurans, prayer beads and time to pray.

"Disrespect for the Holy Quran is not now, nor has it ever been, nor will it ever be tolerated by the United States," Rice said in a statement to a Senate appropriations subcommittee.

"Our military authorities are investigating these allegations fully," she said. "If they are proven true, we will take appropriate action. Respect for the religious freedom for all individuals is one of the founding principles of the United States. ... I am asking that all of our friends reject incitement to violence by those who would mischaracterize our intentions."

In Thursday's bloodiest incident, police fired on hundreds of anti-U.S. demonstrators in the town of Khogyani to prevent them from going to Jalalabad, 20 miles to the north, said police chief Maj. Gul Wali.

Wali counted three deaths among the protesters, who he said were armed.

However, Interior Ministry spokesman Latufallah Mashal said two people died. Mashal said a third protester was killed in a separate clash with police in Wardak province, south of Kabul. He provided no details.

In neighboring Logar province, CARE International, one of the largest international relief groups in Afghanistan, said students attacked its office, thumping one staff member over the head with a piece of wood and trashing two computers. Another foreign relief group office next door was reportedly set ablaze.

"It's the symbols of this change in Afghanistan" that have been singled out, said Paul Barker, the country director for CARE. "There are probably people around the country inciting this."

In the capital, more than 200 young men gathered in front of Kabul University chanting "Death to America!" and carrying banners including one saying: "Those who insult the Quran should be brought to justice."

About two dozen students clambered onto the roof of a nearby building and burned an American flag to applause and cries of "God is great!" from the crowd below. Dozens of police -- some armed with sticks, others with assault rifles -- looked on.

Demonstrations also broke out in Pakistan, where more than 200 supporters of a radical Islamic group rallied in the northwestern city of Peshawar, and demanded that the United States offer an apology.

"This insulting of the Quran is a shameful act. It has torn to bits America's claims of being an enlightened country," said Abdul Jalil Jan, one of the organizers.

Peaceful demonstrations also have been reported in at least five other Afghan provinces and the Pakistani cities of Islamabad and Quetta. A larger demonstration in Pakistan was planned for Friday.