United Nations Secretary-General Ban Ki-moon has condemned the mass killing of civilians in the Damascus suburb of Daraya, his spokesman Martin Nesirky said at a briefing on Monday.

“Ban Ki-moon strongly condemns this appalling and brutal crime,” Nesirky told reporters, adding that the secretary-general has called for an immediate investigation into the incident.

Al Arabiya television reported that more than 300 people were killed in Daraya, citing opposition activists. Most of the bodies were found in the basements of residential buildings.

The opposition alleges that the dead were executed during a military operation by government troops on August 25.

“The secretary-general is of course shocked by these reports,” Nesirky said.

Since it began with demonstrations against President Bashar al-Assad’s regime in March 2011, the Syrian conflict has escalated into a full-scale civil war that has claimed up to 20,000 lives, according to estimates by various Syrian opposition groups.