Another missile stuck nearby in the al-Bab area of Aleppo as residents rushed to carry out the wounded from the bus, the British-based Syrian Observatory for Human Rights said.
Syrian forces have killed more than 500 people, mainly civilians, since Dec. 15 in a bombing campaign of Aleppo, mostly by dropping crude explosive-filled barrels over rebel neighborhoods. The al-Bab area has been a frequent target. The bombing has been one of the most intense attacks on residential areas since the Syrian uprising began three years ago against the rule of President Bashar Assad.
Rebels seized parts of Aleppo, Syria's largest city, during the summer of 2012. Valerie Amos, the United Nations undersecretary general for humanitarian affairs, said Tuesday she was "extremely concerned with the deteriorating humanitarian situation" there.
"The insecurity continues to have a major impact on our efforts to reach people with life-saving assistance," Amos said. "I join U.N. Secretary-General Ban Ki-moon in strongly condemning the attacks against civilians in Aleppo and in many other parts of Syria."
Rebels also have fired at neighborhoods perceived as loyal to Assad in other parts of Syria. On Tuesday, Syrian state media reported that a rebel-fired mortar shell killed three people in the central city of Homs. The Observatory said one person was killed in the same incident. Such conflicting claims are typical.
The Observatory obtains its information from a network of activists throughout the country.
On Tuesday, they also reported that more than 130,000 people have been killed since Syria's uprising began in March 2011. The United Nations said in July that 100,000 Syrians had been killed and has not updated that figure since.
The Observatory said about 66,000 of the dead were civilians, including some 7,000 children. The group said more than 32,000 soldiers have been killed, but said its estimates of rebels killed likely was far less than the actual number killed, because powerful al-Qaida-linked brigades largely do not report their fighters' deaths.
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