Witnesses and at least one government official alleged that U.S. troops threw a grenade into the crowd in Kunar province, but police did not immediately confirm that. The provincial governor said that U.S. troops showed him fragments of a Russian-made grenade, an implication that someone in the crowd caused the blast.
Speaking to CBS News' Fazul Rahim in Kabul, both the International Security Assistance Force-Afghanistan and the U.S. military categorically denied that the grenade was thrown by a U.S. soldier.
Brian Naranjo, a spokesman for the U.S. Forces-Afghanistan, said a coalition vehicle became stuck on the road and while troops were attempting to get it out they came under fire. An insurgent threw a hand grenade which injured 3 U.S. soldiers, one Afghan soldier and a 12-year-old Afghan boy, according to Naranjo.
He added that the military did not have any reports of further injuries or deaths.
A hospital official who goes by one name, Dr. Afizullah, said 54 people were wounded in the blast and many were children from a nearby school. Education officials said about 15 children were wounded.
Kunar Gov. Sayed Fazelullah Wahidi said that three Americans were wounded in the incident, one seriously.
The explosion took place in a crowded bazaar near a school and mosque. Afghan officials said the American convoy had broken down because of a flat tire and a crowd had gathered.
Kunar's provincial police chief, Abdul Jalal Jalal, said someone threw a grenade into the crowd and officials were investigating the assailant's identity.
Suspicion among Afghans immediately fell on the Americans. An eyewitness in the bazaar, Abullah Sadat, told The Associated Press by telephone that a U.S. soldier had thrown a grenade.
Mohammad Asif Nang, the spokesman for the country's education ministry in Kabul, also blamed the Americans for throwing a grenade. He said he got his information from officials in Kunar.
Civilian casualties have long been a source of tension between the Afghan government and U.S. and NATO troops. President Hamid Karzai has pleaded with U.S. officials to reduce the number of civilian casualties.
The Pentagon said Monday that U.S.and procedures during an air assault on Taliban fighters last month which killed a disputed number of civilians.
Pentagon spokesman Geoff Morrell said the number of Taliban militants killed in the May 4 air strikes "greatly outnumbered" the number of civilians slain. But Morrell noted some problems in the way the strikes were carried out, citing a U.S. warplane that appears to have erred in bombing its target.
American officials say that Taliban militants purposely try to cause civilian casualties that can be blamed on U.S. forces as a propaganda tool to turn ordinary Afghans against the international military effort.