Pro-government forces - backed by Russia - intensified their efforts to retake the last major rebel stronghold on Sunday night, leading to one of the heaviest bombardments in seven years of war. Since then, activists say more than 60 children have died and at least 1,400 people have been injured, and another hospital has been put out of service. Fresh regime air strikes on Wednesday killed at least 24 people raising the total death toll in three days to at least 296, among them dozens of children,the UK-based Syrian Observatory for Human Rights said. Bombardment on Tuesday killed 106 civilians, including 19 children, the Britain-based war monitor said. It was the second straight day that the civilian death toll topped 100, after 127 were killed Monday in Eastern Ghoutas bloodiest day in four years. Pro-regime forces fired rockets and dropped barrel bombs - containers filled with explosives and shrapnel - from helicopters on the towns and villages of the rural district just outside Damascus, where rebels fighting dictator Bashar al-Assad have their last big redoubt near the capital. On the outskirts of Damascus, air strikes, rockets and artillery fire have been battering the Eastern Ghouta enclave in apparent preparation for a regime ground assault. The strikes left an important hospital out of action, further limiting the little medical aid that besieged civilians can access. "The Arbin hospital was hit twice today and is now out of service," said Mousa Naffa, country director in Jordan for the Syrian American Medical Society (SAMS), which supported the clinic. The Observatory blamed Russian warplanes, saying Moscow carried out its first strikes in three months on Eastern Ghouta. The rebel-held region is nominally included in a "de-escalation" deal meant to tamp down violence, but Assad appears to be preparing troops for a ground assault to retake it. UN Secretary-General Antonio Guterres was "deeply alarmed by the escalating situation in Eastern Ghouta and its devastating impact on civilians," said spokesman Stephane Dujarric. US State Department spokeswoman Heather Nauert criticized the "siege and starve tactics" of the Assad regime and said: "The cessation of violence must begin now."Targeting of Hospitals Eastern Ghouta is home to more than 400,000 people living under crippling regime siege, with little access to food or medical resources. The United Nations said six hospitals had been hit in the region in the past 48 hours, in addition to the one in Arbin. At least three were out of service and two were only partially functioning, said the UNs regional humanitarian coordinator for Syria, Panos Moumtzis. "Its beyond imagination what is happening in East Ghouta today," he said. "The untold suffering is intolerable and residents have no idea whether they will live or die. This nightmare in East Ghouta must end and must end now." Hours before the Arbin hospital was bombed, a doctor there spoke of the casualties they had been treating. "February 19 was... one of the worst days that weve ever had in the history of this crisis," Abu al-Yasar told AFP. He described treating a one-year-old with blue skin and a faint pulse, rescued from under the rubble. "I opened his mouth to put in a breathing tube and I found it packed with dirt," said Abu al-Yasar. He pulled out the dirt as fast as possible, put in the breathing tube and managed to save the baby. "This is just one story from among hundreds of wounded." The bloodshed prompted the UN childrens agency UNICEF to issue a largely blank statement saying "we no longer have the words to describe childrens suffering." UN officials have repeatedly called for immediate cessation of hostilities in Ghouta and the resumption of aid shipments. Syrias main opposition group condemned the government onslaught as a "bloodbath" and a "war crime", saying it may pull out of UN-backed peace talks in protest.