The air raids struck numerous towns controlled by Jaish Khalid ibn al-Walid, an Islamic State affiliate and rival of the allied Syrian and Russian governments as well as Syrian rebel groups, the monitor said.
WATCH: Syrian rebels begin evacuating border with Israeli-controlled Golan Heights
The NGO says Jaish Khalid ibn al-Walid is using some 30,000 civilians as human shields.
In addition to claiming 26 lives, the aerial bombardments reportedly destroyed entire neighbourhoods and caused extensive damage to infrastructure.
The report comes just hours after the United Nations High Commissioner for Refugees appealed for warring parties in Syria to provide safe passage for an estimated 140,000 civilians displaced by fighting in southwestern Syria.
WATCH: Assad presses assault in southwest Syria, civilians flee
The UNHCR said it was prepared to work with Syria and Russia to set up support centres for refugees.