At least 250 people have been killed over two days of escalating violence in eastern Ghouta as Syrian and Russian airstrikes continue to bombard the rebel-held enclave.
Rockets and artillery fire have been targeting the Damascus suburb in an apparent preparation for a government ground assault.
The UN has described the situation as "beyond imagination", while Amnesty International said "flagrant war crimes" were being committed.
In a major development in Syria's complex seven-year war, forces loyal to Bashar al Assad were sent to the northern Afrin region, where they came under fire by Turkish forces attacking the Kurdish-controlled area.
A spokesman for Turkey's Prime Minister Recep Tayyip Erdogan said there would be "serious consequences" after a convoy of about 50 vehicles tried to enter Afrin on Tuesday but were repelled by artillery fire.
The bombardment in eastern Ghouta has led to the worst 48-hour death toll in Syria since a chemical attack in 2013.
The Syrian Observatory for Human Rights said 106 civilians, including 19 children, were killed in the violence on Tuesday.
It was the second straight day that the civilian death toll topped 100 after 127 people were killed Monday in eastern Ghouta's bloodiest day in four years.
The strikes left an important hospital out of action, further limiting the little medical aid that besieged civilians can access.
The Observatory blamed Russian warplanes, saying Moscow carried out its first strikes in three months on eastern Ghouta.
Sky News
