Syrian government warplanes on Sunday pounded an area near Damascus controlled by militant groups, according to footage released by state TV.
The latest development came as President Bashar al-Assad's forces stepped up efforts to remove militants from an area near the government-controlled capital.
Thick clouds of smoke rose from the al-Hajar al-Aswad area, and the sound of jets could be heard in broadcasts from the area just south of Damascus that is controlled by fighters from the Islamic State and the former Nusra Front groups.
UNRWA, the UN agency that cares for Palestinians, said two Palestinian refugee civilians – a father and son – had been killed in their home in the nearby Palestinian camp of Yarmouk.
Thousands of homes had been destroyed in Yarmouk in the past four days of fighting, UNRWA spokesman Chris Gunness said.
"There must be safe passage for the sick and the wounded and the dying civilians," he said. "Yarmouk has been transformed into a death camp, like one of the lower regions of hell."
The militant-held enclave is next to a pocket held by rebel groups fighting under the Free Syrian Army banner.
Assad, backed by Iran and Russia, is seeking to crush the last few besieged rebel and militant enclaves, building on the defeat of militants in the eastern Ghouta region, which was their last major stronghold near the capital.
Militants on Saturday began to withdraw from an enclave they held northeast of Damascus in the eastern Qalamoun region in a surrender agreement with the government. They are being transported to opposition-held territory at the Turkish border.
Although the conquest of eastern Qalamoun and the enclave south of Damascus will leave just one remaining besieged anti-government enclave, north of the city of Homs, large parts of Syria at the borders with Jordan, Israel, Turkey and Iraq remain outside Assad's control.
Anti-Assad fighters hold a chunk of territory in the southwest and the northwest, and Kurdish-led militias, backed by the United States, control an expanse of northern and eastern Syria.