The medical charity Doctors Without Borders says at least 70 people were killed in an airstrike and shelling on a crowded, rebel-held neighborhood near the Syrian capital on Friday.
The group, known by its French acronym MSF, said Saturday that the attack on a marketplace in the Douma neighborhood near Damascus wounded another 550 people.
According to the group, a nearby MSF-supported hospital helped treat the first wave of victims.
In a statement, Doctors Without Borders did not assign blame for what it called "an extremely violent bombing." An MSF spokesman, Tim Shenk, told VOA the Douma area has been under sustained attack from Syrian forces for years. Opposition fighters have regularly traded fire with the Syrian military in Douma, which lies within a major rebel stronghold.
Turkey flies anti-IS sorties
In other news, Turkish jets on Saturday launched bombing raids against Islamic State targets in Syria. A senior Turkish official told the Reuters news agency that American jets were standing by to assist.
Turkey vowed to take a more active role in combating IS in July, as part of a multipronged offensive that also involved Turkey ramping up attacks on Kurdish militants.
The Syrian conflict has killed an estimated 250,000 people since it began in 2011.