Those forces were trying to join Kurdish militias defending Afrin from Turkish troops who crossed the border and were advancing from the north. They retreated after Turkish jets and artillery bombarded them, the Turkish government said.
The government’s move to support the Kurds threatened to unravel months of diplomatic efforts by Russia, Turkey and Iran to de-escalate the conflict. It also signaled a new phase of the war with a greater potential for military engagements between other countries with a stake in the outcome, among them Turkey, Iran and the United States.
In the Ghouta area, Monday was the deadliest day in three years, according to the Syrian Observatory for Human Rights, a Britain-based war monitoring group. The Union of Medical Care and Relief Organizations put the two-day death toll at 200 or more.
Eastern Ghouta, a cluster of concrete-block towns and farmland with an estimated population of nearly 400,000, is one of the last major areas held by insurgents fighting the government of President Bashar al-Assad. Rebels based there periodically shell government-held neighborhoods of Damascus.
As the government airstrikes have stepped up, so has the rebel shelling. On Tuesday, another 12 people were killed and dozens were wounded.
Five medical facilities in rebel-held eastern Ghouta were damaged in government attacks overnight, and several medical workers were killed, according to doctors working at hospitals supported by the Syrian American Medical Society.
“Such targeting of innocent civilians and infrastructure must stop now,” Panos Moumtzis, the United Nations regional humanitarian coordinator for the Syria crisis, said in a statement.
The Syrian government and Russia have escalated an aerial campaign to subdue the rebel-held area, which has been besieged for years. And pro-government forces were gathering nearby for a possible ground assault. General Hassan’s video showed military vehicles said to be massing nearby.
Residents described the events as more like an all-out attack on civilians and infrastructure to force a surrender, a tactic used in previous battles across Syria. The government claims that there are few civilians in eastern Ghouta and that those who remain are being held as human shields.
“We are still alive, we can’t walk outside the house, even a few meters,” Mr. Dimashqi said.
Referring to his daughter, whom he calls Loulou, after the 1950s actress Gina Lollobrigida, he said, “I have only this baby, and we can’t find food for her.”
“We have no other choice except resisting until the last moment,” he said. “Death and life became equal to me.”
Many in eastern Ghouta have been hiding in shelters. Shadi Jad, the father of a three-week-old, said the infant had not seen the sun or breathed fresh air in 48 hours.
“We have a small window in our shelter,” he said. “I bring him close to the window just for seconds, to get some warmth from the sunlight.”
Wassim Khatib, a resident reached by video chat, said that recent negotiations for hard-line insurgents to leave the area had failed.
“Civilians were never allowed to leave,” he said, looking pale and exhausted. “If the regime wanted the civilians to be evacuated, they could have announced that or at least dropped leaflets.”
Until last year, tunnels provided a way for goods and people to enter and leave the besieged suburb, but smuggling fees were always high. Movement has become nearly impossible since government forces took over surrounding territory. Roads were opened briefly, but only government employees were allowed to go back and forth, residents said.
The government-run news agency Sana said that reports of an escalation in eastern Ghouta were “lies, deception, and fabrications” used by “terrorist organizations and their sponsors within capital cities conspiring against Syrians.”
Mr. Moumtzis of the United Nations called on all parties to “strictly adhere to their obligations under international humanitarian law to take all feasible measures to protect civilians from harm, including the prohibition on launching of indiscriminate attacks and principles of proportionality and precautions.”
“The humanitarian situation of civilians in East Ghouta is spiraling out of control,” he said. “It’s imperative to end this senseless human suffering now.”
No Syrian government forces were involved in the assault on Afrin, but the attack drew a sharp rebuke from Turkey, which fired an artillery barrage at the advancing militia forces. The militias appeared to have halted their advance in the face of Turkish bombardment.
People’s militias, a mixture of pro-government tribal volunteers from eastern Syria and Shiite groups from northern Syria supported by Iran were shown on Syrian state television crossing the administrative border of Afrin. Dozens of vehicles passed, loaded with fighters making victory signs and flying government flags.
One government supporter filmed what he said were Turkish spotter planes and black plumes of smoke rising in the distance from artillery bombardment.
The advance by pro-government forces into Afrin came after a Kurdish official announced an agreement had been made for the Syrian government to support the Kurdish militias in Afrin against advancing Turkish forces.
Yet Turkey seemed to have been surprised by the agreement and the swift advance of the pro-government militias, and fired artillery strikes to stop their advance on Tuesday.
Turkey’s president, Recep Tayyip Erdogan, said the government advance on Afrin was halted after he spoke with President Vladimir Putin of Russia by telephone Monday. He also spoke with Iran’s president, Hassan Rouhani. Mr. Erdogan announced that the military operations against Afrin would continue, partly so that hundreds of thousands of Syrian refugees in Turkey could return home.
“We haven’t gone there to burn down what comes in front of us,” Mr. Erdogan said in a speech to his party’s lawmakers at the Parliament. “We entered there to make it a safe, livable place for those hundreds thousands of people who still live in our country.”
“In the following days, the siege of Afrin will take place more swiftly,” he added.