More than 70 people have reportedly been killed and hundreds wounded as airstrikes, rocket fire and shelling by Syrian government forces and their allies hit a rebel enclave in eastern Ghouta.
It comes as pro-regime troops are expected to enter Afrin before the end of Monday to help the Kurdish YPG militia try to repel a month-old Turkish military assault.
The Syrian state news agency SANA said: "Popular forces will arrive in Afrin within a few hours to support its people's stand against the Turkish regime's attack on the area and its people.
"This comes in the framework of supporting residents and defending the territorial unity and sovereignty of Syria."
Turkey's deputy prime minister Bekir Bozdag responded to the announcement of the forces by warning their arrival would give the "green light" to a divided Syria.
The foreign minister Mevlut Cavusoglu said if the Syrian army came to defend the YPG "nothing and nobody can stop Turkish soldiers".
The British-based Syrian Observatory for Human Rights claimed airstrikes, rocket fire and artillery across eastern Ghouta had killed 71 in the space of 24 hours, though other agencies said the figure was closer to 44.
The Observatory said 325 were wounded.
Among the dead, 20 were killed in airstrikes on Hammuriyeh and nine others in a bombardment on Saqba.
Four children were killed.
"The regime is bombing eastern Ghouta to pave the way for a ground offensive," said Observatory head Rami Abdel Rahman.
There was no immediate comment from the Syrian military. The Damascus government has said it only targets militants.
The sound of the aircraft prompted Hammuriyah's residents to rush inside in a panic.
Alaa al-Din, a 23-year-old Syrian in Hammuriyeh, said civilians were afraid of a potential government offensive.
"Ghouta's fate is unknown. We've got nothing but God's mercy and hiding out in our basements," he told AFP on Monday.
"There's no alternative."
Afrin, on the border of Turkey, is held by the Kurdish People's Protection Units (YPG). Turkey sees their presence as a direct threat and launched Operation Olive Branch to pound the region.
The offensive has not been well-received by everyone, and officials have detained 786 people who have protested or criticised the military offensive.
Authorities renamed the street where the US embassy is based, calling it Olive Branch Street, instead of Nexvat Tandagon Street.
The US embassy said it was up to Turkish authorities to decide street names.
Turkish President Recep Tayyip Erdogan spoke to Russian counterpart Vladimir Putin on Monday afternoon, in a call where they confirmed their commitment to fighting "terrorism" in Syria.