KABUL, Afghanistan (AP) -- Afghans angry over the burning of a Quran at a small Florida church stormed a U.N. compound in northern Afghanistan on Friday, killing seven foreigners, including four Nepalese guards.

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Afghan authorities suspect insurgents melded into the mob and they announced the arrest of more than 20 people, including a militant they suspect was the ringleader of the assault in Mazar-i-Sharif, the provincial capital of Balkh province. The suspect was an insurgent from Kapisa province, a hotbed of militancy about 250 miles (400 kilometers) southeast of the city, said Rawof Taj, deputy provincial police chief.

The topic of Quran burning stirred outrage among millions of Muslims and others worldwide after the Rev. Terry Jones' small church, Dove Outreach Center, threatened to destroy a copy of the holy book last year. The pastor backed down but the church in Gainesville, Florida, went through with the burning last month.

Four protesters also died in the violence in Mazar-i-Sharif, which is on a list of the first seven areas of the country where Afghan security forces are slated to take over from the U.S.-led coalition starting in July. Other demonstrations, which were peaceful, were held in Kabul and Herat in western Afghanistan, fueling resentment against the West at a critical moment in the Afghan war.

Protesters burned a U.S. flag at a sports stadium in Herat and chanted "Death to the U.S." and "They broke the heart of Islam." About 100 people gathered at a traffic circle near the U.S. Embassy in Kabul. One protester carried a sign that said: "We want these bloody bastard Americans with all their forces to leave Afghanistan."

Initially, Afghan police reported that eight foreigners had been killed in Mazar-i-Sharif.

Late Friday, Dan McNorton, a spokesman for the United Nations Assistance Mission in Afghanistan in Kabul, revised the death toll to seven - four foreign security guards and three other foreigners.

The guards were from Nepal, according to Gen. Daud Daud, commander of Afghan National Police in several northern provinces.

Sweden Foreign Minister Carl Bildt said Joakim Dungel, a 33-year-old Swede who worked at the U.N. office, was among those killed.

Norwegian Defense Ministry spokeswoman Maj. Heidi Langvik-Hansen said Lt. Col. Siri Skare, a 53-year-old female pilot working for the U.N., died in the attack.

The nationality of the remaining victim was not known.

The Russian Foreign Ministry said the head of the mission in Mazar-i-Sharif, a Russian citizen, was injured in the attack, but not seriously.

Police who went to investigate, said the U.N. compound was littered with broken glass and bullet casings.

Abdul Karim, a police officer in the city, said he saw the bullet-riddled bodies of three Nepalese guards lying in the yard and a fourth on the first floor.

He said another victim with a serious head wound died on a stairway to the basement of the compound. A man who was killed inside a room had severe wounds to his face and body, Karim said.

Munir Ahmad Farhad, a spokesman in Balkh province, said the protest began peacefully when several hundred demonstrators gathered outside the U.N. mission's compound, choosing an obvious symbol of the international community's involvement in Afghanistan to denounce the Quran's desecration. It turned violent when some protesters seized the guards' weapons and started shooting, then the crowds stormed the building and set fires that sent plumes of black smoke into the air, he said.

One protester, Ahmad Gul, a 32-year-old teacher in the city, gave a different account. He said the protesters disarmed three guards to prevent any violence from breaking out. Associated Press video showed protesters banging AK-47 rifles on the curb, breaking them into pieces. He said the protesters were killed and wounded by Afghan security forces.

"I disarmed three guards myself and we took out the bullets," Gul said, sternly shaking his finger as he shouted. "With my eyes, I saw them (Afghan security forces) kill two and wound 10." As he talked, he became increasingly indignant and he started shouting: "Death to America!" "We are going to fight."

The United Nations Security Council scheduled an emergency meeting on the deadly attack, which drew condemnations from around the world.

Secretary-General Ban Ki-moon, who is in Nairobi, said it was "a cowardly attack that cannot be justified under any circumstances."

He said he had instructed the top U.N. official in Afghanistan, Staffan de Mistura, to assess the situation and take any "necessary measures to ensure the safety of all U.N. staff."

President Barack Obama condemned the attack and underscored the importance of the U.N.'s work in Afghanistan.

"We stress the importance of calm and urge all parties to reject violence and resolve differences through dialogue," Obama said.

At the U.S. State Department, spokesman Mark Toner said the burning of a Quran in Florida was contrary to Americans' respect for Islam and religious tolerance. "This is an isolated act done by a small group of people and ... does not reflect the respect the people of the United States have toward Islam," he said.

The church's website stated that after a five-hour trial on March 20, the Quran "was found guilty and a copy was burned inside the building." A picture on the website shows a book in flames in a small portable fire pit. The church on Friday confirmed that the Quran had been burned.

In a statement, Jones did not comment on whether the church's act had led to the deaths. Instead he said it was time to "hold Islam accountable" and called on the United States and the U.N. to hold "these countries and people accountable for what they have done as well as for any excuses they may use to promote their terrorist activities."

Last week, Afghan President Hamid Karzai issued a statement calling the burning a "crime against a religion." He denounced the U.N. attack as a "disrespectful and abhorrent act" and called on the U.S. and the United Nations to bring to justice those who burned the holy book. Karzai issued a statement late Friday calling the killings an "inhumane act" that was "against the values of Islam and Afghans." He said he planned to call officials at U.N. headquarters to express his regret and condolences from the people of Afghanistan.

The U.N. has been the target of previous attacks.

In October 2010, a suicide car bomber and three armed militants wearing explosives vests and dressed as women attacked a U.N. compound in Herat in western Afghanistan. Afghan security forces killed the attackers and no U.N. employees were harmed. In October 2009, Taliban militants attacked a guesthouse used by United Nations workers in central Kabul. Eight people were killed, including five foreigners working for the U.N.

Separately, the U.S. Department of Defense announced that six U.S. Army soldiers were killed in separate incidents in fighting against insurgents during an operation in eastern Kunar province, which neighbors Pakistan's lawless tribal areas. Insurgents have slowly been filtering back into Afghanistan from safe havens in Pakistan as the spring fighting season gets under way.

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Associated Press writers Amir Shah in Kabul, Edith M. Lederer at the U.N. and Mitch Stacy in Tampa, Florida, contributed to this report.