Heavy Syrian bombardment killed at least 77 civilians in rebel-held Eastern Ghouta on Monday, a monitor said, as regime forces appeared to prepare for an imminent ground assault.
The escalation came as pro-government forces were also expected to enter the northern Kurdish-controlled enclave of Afrin, to take a stand against a month-old Turkish assault there.
Held by rebels since 2012, Eastern Ghouta is the last opposition pocket around Damascus and President Bashar al-Assad has dispatched reinforcements there in an apparent concerted effort to retake it.
As the United Nations decried the "senseless human suffering", a barrage of air strikes, rocket fire and artillery slammed into several towns across Eastern Ghouta on Monday.
The Britain-based Syrian Observatory for Human Rights said at least 20 children were among 77 civilians killed in the assault, while around 300 other people were wounded.
"The regime is bombing Eastern Ghouta to pave the way for a ground offensive," said Observatory head Rami Abdel Rahman.
The main opposition National Coalition, which is based in Turkey, denounced the "war of extermination" in Eastern Ghouta as well as the "international silence."
In a statement, it also accused regime ally Russia of seeking to "bury the political process" for a solution to the conflict.
Residents of Hammuriyeh could be seen rushing indoors in panic as soon as they heard the sound of jets.
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.
"There's no alternative."
Shelling also hit the town of Douma, where an AFP correspondent saw five toddlers brought to a hospital, covered in dust and wailing uncontrollably.
The hospital was full of distraught civilians: one father slapped his forehead after finding his two dead children, another erupted into tears as he discovered the body of his newborn on a purple sheet next to a pool of blood.
Eastern Ghouta is held by two main Islamist factions, while jihadists control small pockets including one directly adjacent to the capital.
The Observatory and Syrian daily newspaper Al-Watan had said negotiations were under way for the evacuation of jihadists from Eastern Ghouta.
But escalating military pressure indicate that the regime would opt for a ground assault instead of talks, the monitor said.
Government troops carried out a relentless five-day bombing campaign earlier this month that killed around 250 civilians in the enclave and wounded hundreds.
Around the same time, the monitor said, the regime began dispatching military reinforcements to Eastern Ghouta.
After days of relative calm, the government sent more than 260 rockets crashing into Eastern Ghouta on Sunday.
The regime is keen to regain control of Eastern Ghouta to halt the deadly salvo of rockets and mortars that rebels fire on Damascus.
The United Nations said Monday that the targeting of civilians in Eastern Ghouta "must stop now."
"It's imperative to end this senseless human suffering," Panos Moumtzis, the UN's Regional Humanitarian Coordinator for the Syria Crisis said in a statement.
About half a dozen rockets hit the capital Sunday night, AFP correspondents said. State news agency SANA reported that one person was killed.
More than 20 civilians have been killed by rebel fire this month alone in regime-held Damascus.
Regime to enter Afrin?
All was quiet in the capital on Monday but since rumours of an imminent assault on Eastern Ghouta started spreading, people living close to the rebel enclave started packing their bags.
Jawad al-Obros, 30, said he was looking to move to a hotel in the western sector of the city to escape his home in an east Damascus neighbourhood that has been regularly hit by rockets from Ghouta.
"We're tired of this situation. It seems that there's no solution but a full-blown military one," he told AFP.
More than 340,000 people have been killed since Syria's conflict erupted in 2011 with protests against Assad's government.
It has since evolved into a war that has carved up the country into rival zones of influence among the regime, rebels, jihadists and Kurdish forces.
The Kurdish People's Protection Units (YPG) control the northwestern region of Afrin, target of a month-old assault by the Turkish army and allied Syrian rebels.
Turkey sees the YPG as a "terror" group linked to the Kurdistan Workers Party (PKK), outlawed by Ankara, and wants to clear it from its southern border.
The YPG has controlled Afrin since 2012, when Syrian troops withdrew from it and other Kurdish-majority areas.
Syrian state media said Monday that pro-regime forces were preparing to enter the area to "join the resistance against the Turkish aggression."