Russian and Syrian government forces have killed at least 136 civilians over the past 48 hours in Eastern Ghouta, a suburb of the capital Damascus, according to residents and monitoring groups.
On Monday, 30 civilians were killed in air raids; on the following day, 80 more were killed, and on Wednesday, another 26 were killed.
At least 22 children and 21 women were among the dead.
"Scenes of entire buildings, housing whole families crashing down with the residents - women, children and men - still inside have become a frequent image," Abu Salem al-Shami, a resident and activist, told Al Jazeera by phone from Eastern Ghouta, an area that has been under opposition control since 2013.
The Syrian Observatory for Human Rights (SOHR), a UK-based monitoring group, says Tuesday marked the "largest massacre in Syria" since April's chemical attack on Khan Sheikhoun in Idlib province when more than 80 people were killed.
Eastern Ghouta's proximity to the capital - where the government of President Bashar al-Assad resides - renders it a key target for the government and its principal ally, Russia.
Assad's forces have enforced an ongoing military siege on the area since 2013 in an attempt to drain the armed opposition groups there.
While Eastern Ghouta, home to some 400,000 people, has been under constant bombardment since then, there has been an uptick in air raids in recent months as it is one of the last remaining opposition strongholds in Syria.
"People on the outside think that Russia and the Syrian regime are killing armed fighters, but this is completely false. Only civilians are being targeted - regular civilians, the people of Damascus," al-Shami, who lost 10 of his immediate family members when their building was attacked last April, said.
According to the SOHR, 369 people have been killed in Eastern Ghouta, including 91 children and 68 women, since the end of December.
Source: Agencies