A bombardment of eastern Ghouta has led to the worst 48-hour death toll in Syria since a chemical attack in 2013.
Since Sunday night 250 people have been killed, according to the Syrian Observatory for Human Rights.
It reported that 106 of those were killed in the bombardment on Tuesday.
Fears are mounting that violence across the country will escalate and the United States has said it is "deeply concerned" about the situation.
State Department spokeswoman Heather Nauert said the "cessation of violence must begin now."
Meanwhile, UN envoy Staffan de Mistura said: "This has a risk of becoming a second Aleppo, and we have learned, I hope, lessons from that."
The UN children's agency, UNICEF, condemned the killings in Ghouta, issuing a blank statement under a headline saying: "Do those inflicting the suffering still have words to justify their barbaric acts?"
UNICEF said: "We no longer have the words to describe children's suffering and our outrage."
Statement from @gcappelaere on the war on children in #Syria
Reports of mass casualties among children in Eastern #Ghouta and Damascus#ChildrenUnderAttack pic.twitter.com/X2FYJ4OPnf
— UNICEF MENA (@UNICEFmena) February 20, 2018
The bombardment saw the use of war planes, helicopter gunships, missiles and artillery.
It came as a convoy of militia fighters, waving weapons and chanting "one Syria," drove into the Kurdish enclave of Afrin on Tuesday and were immediately met with shellfire.
Turkish President Recep Tayyip Erdogan said the convoy was made up of "terrorists" and repeated earlier reports that shellfire from Turkish forces led to a retreat of pro-regime fighters.
Turkey, with the support of some Syrian rebels, has waged a campaign against Kurdish militants, whom Ankara regards as terrorists.
Turkey wants to drive the Kurdish YPG militia from Afrin because it considers it a big security threat on its border.
The campaign pits the Turkish army directly against the military alliance backing President Bashar al Assad's government, further complicating the Syrian crisis.
According to Turkish state media, the pro-government forces retreated by 10km (6m), from Afrin city.
But Syrian television showed them passing through a checkpoint with the insignia of the Kurdish security force.
Syrian state media reported that Turkey fired at its forces on Tuesday as they entered Afrin.
SANA said: "Turkish regime forces targeted the locations of popular forces with artillery fire as they arrived to the Afrin region."
Turkey said these were "warning shots." The Kurdish militia denied it had retreated.
Meanwhile, in rebel-held areas near Damascus, nearly 130 people have died in the last two days, according to paramedics, amid heavy Syrian government shelling and airstrikes.
Eight people died in Damascus when retaliatory shells rained down, and panic spread among residents of the Syrian capital.
The bombardment has been some of the deadliest in the region in three years.
