Ralib al-Omar of the Yusif al-Asma rebel group told CNN the attack occurred at the Dar al-Shifa Hospital.
The Syrian Network for Human Rights in London issued a statement alleging 74 people had been killed across Syria, including 27 in Aleppo. It wasn't known if that number included those killed in the hospital strike.
CNN noted claims by the Syrian opposition and government cannot be independently verified because of restrictions imposed on journalists.
An estimated 37,000 people have died in the 20 months since protests began against the government of President Bashar Assad.
In other related developments Wednesday, Syrian neighbor Turkey asked its NATO allies for Patriot missiles Wednesday to bolster its border defenses.
A Turkish government statement said the additional weapons were needed because of "the threats and risks posed by the continuing crisis in Syria to our national security." NATO took the request under advisement.
Gulf News reported George Sabra, president of the Syrian National Council and member of the National Coalition of the Syrian Revolution and Oppositions Forces, said Wednesday the price tag for rebuilding the country after the civil war ends would be high.
"In the first six month after the collapse of the Al Assad regime there will be a need of $60 billion to rebuild Syria," he said.
"We haven't get any loan yet but we will ask for this from the friends of Syria."
Sabra said Syrian businessmen around the world have promised to commit more than $5 billion so far.