Kabul — A suicide bomber detonated his explosive vest at a military base in eastern Afghanistan on Wednesday, killing eight Americans, U.S. officials said.



The explosion occurred at Forward Operating Base Chapman in Khost province near the Afghan border with Pakistan.



State Department spokesman Ian Kelly confirmed that eight Americans died in the attack.



"We mourn the loss of life in this attack, and are withholding further details pending notification of next of kin," he said.



In Kabul, a spokesman for the international coalition force in Kabul said no U.S. or NATO troops were killed in the afternoon explosion at Chapman, one of dozens of forward operating bases that support reconstruction efforts and other civilian operations across the nation.



An attacker wearing a suicide vest caused the explosion, according to a senior U.S. official in Washington. Another senior U.S. official in Washington said there were conflicting reports on the number of casualties, but that others were injured in the attack. The officials spoke on condition of anonymity because not all details about the incident had been confirmed.



Wazir Pacha, a police spokesman in Khost province, said local people reported hearing a blast on the base where an explosion in January killed an Afghan civilian and wounded four others. Soon afterward, two helicopters landed, a police officer in Khost said.



Separately on Wednesday, NATO questioned Afghan reports that international troops killed 10 civilians, including schoolchildren, in a weekend attack that prompted hundreds of angry Afghan protesters to burn an effigy of U.S. President Barack Obama and chant "death" to America.



The head of an investigative team appointed by Afghan President Hamid Karzai told the Associated Press by telephone that eight students between the ages of 12 and 14 were among the dead discovered in a village house in a remote section of Kunar province in eastern Afghanistan. NATO said in a statement released late Wednesday night that while there was no direct evidence to substantiate the claims, the international force had requested and welcomed a joint investigation to reach an "impartial and accurate determination" of what happened in the attack.



Conflicting accounts of what occurred during fighting in Kunar's Narang district prompted an emotional outcry over civilian deaths, one of the most sensitive issues for international troops fighting the more than eight-year-old war. Although insurgents are responsible for the deaths of far more civilians, those blamed on coalition forces spark the most resentment and undermine the fight against militants. With 37,000 more U.S. and NATO troops being deployed to the battle zone, concern over civilian casualties is unlikely to ease anytime soon.



Several hundred Afghans demonstrated in the capital of Kabul and in the eastern city of Jalalabad where the likeness of Obama, adorned with a small American flag, burned on a pole held above demonstrators.



In Kabul, protesters carried signs that read: "Does peacekeeping mean killing children?" and "Stop killing us." A protester with a bullhorn called on Obama to "take your soldiers out of Afghanistan."



Qari Hamidullah, a student protest leader in Jalalabad, urged the Afghan government to call for the withdrawal of foreign forces from Afghanistan.



"If they do not accept our demand, we will put down our pens, take rockets and got to the mountains to fight the Americans and their forces," Hamidullah told protesters, who chanted and waved their arms in the air.



Karzai said in a statement that he talked to the relatives of the Kunar victims to express his condolences and pledge to bring to justice those responsible for the attack.



Asadullah Wafa, a senior adviser to Karzai who led a 10-member investigative team to Kunar province, said he was convinced that all those killed were innocent civilians.



"I have talked to the principal of the school in the village and he gave us details about the killed children," Wafa said. "The schoolchildren cannot be al-Qaida. I confirm they are innocent people killed by mistake. I talked to Karzai about the findings."



The bodies had already been buried by the time Wafa's team arrived on Tuesday.



Wafa said the villagers demanded that informants "who gave the wrong target to the Americans must be found and punished by a court." Rival clans in Afghan villages have been known to try to settle scores against rival tribes by giving false reports on insurgent activity.



According to the NATO statement, the initial review by Wafa's delegation "asserted that the dead were unarmed civilians removed by international forces from their homes and shot."



The international force's account of what occurred was much different.



NATO said that on Saturday, a joint coalition and Afghan security force entered the village of Ghazi Khan looking for an insurgent group responsible for a series of violent attacks in the area.



"As the joint assault force entered the village, they came under fire from several buildings and in returning fire killed nine individuals," the NATO statement said. "Several assault rifles, ammunition, and ammonium nitrate used in bomb-making were discovered."



Col. Wayne Shanks, spokesman for NATO's International Security Assistance Force in Afghanistan, said at a news conference Wednesday that the coalition force goes to great lengths to avoid civilian casualties.



"In fact, you can see that our enemy, the insurgents, have very little regard for the Afghan people," he said. "We have noticed a very dramatic increase in civilian casualties caused by roadside bombs, by attacks that insurgents have on the Afghan people."



The latest figures released by the United Nations show that 2,021 civilians died during clashes in the first 10 months of this year, up from 1,838 for the same period last year. International forces' stepped up efforts to protect the population has reduced civilian casualties. Taliban insurgents were blamed for 68 percent of the deaths this year — three times more than NATO forces, according to the U.N.



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