A top UN adviser on humanitarian aid for Syria says he believes the frontier with the Israeli-controlled Golan Heights is "hermetically closed" as tens of thousands of civilians flee new fighting in southwestern Syria. Jan Egeland says new fighting in the Deraa and Quneitra regions has led to the discontinuation of what previously had been an "extremely effective lifeline across the border" from Jordan in recent days. Speaking Thursday, he said: "I think you have to ask the Israelis whether they are going also to take part in giving shelter, protection, to people who flee." He says five medical facilities have been hit in Deraa, and that the area targeted by the latest regime offensive is home to some 750,000 civilians "increasingly fleeing for their lives." A barrage of airstrikes hit rebel-held areas in southwestern Syria on Thursday, killing at least 17 civilians, including children, hiding in an underground shelter as government forces pressed their offensive to reclaim a region that was until recently part of a US-backed and negotiated truce. The truce unraveled in recent weeks, triggering a wave of displacement within the southwestern Deraa province and along the border with Jordan. Aid groups have urged Jordan to allow Syrians fleeing the violence into the country. The Britain-based Observatory for Human Rights described Thursdays airstrike in the town of al-Musayfrah in eastern Deraa as the worst violence since the regime offensive began in the area on June 19. Syrian regime troops are seeking to dislodge rebels who have been in the area for years and gain control of the commercial border crossing with Jordan. The Observatory said the airstrike was part of a barrage of missiles that hit the area as regime troops head toward southern Deraa and the border. In recent days, regime forces have gained new ground from rebels, bisecting rebel-held territories in Deraas east and west and severing their supply lines. The Syrian Civil Defense, a team of first responders, said that more than 150 airstrikes targeted 12 towns and villages in eastern and western Deraa since dawn, setting off a new wave of displacement. Up until Tuesday, the United Nations had estimated that nearly 50,000 people are on the move in Daraa, fleeing the violence, most of them heading to villages near the Jordanian border. The Observatory said at least five children were among the 17 killed in the al-Musayfrah strike. Activist with the opposition-operated Horan Free Media, who goes by the name Abu Mahmoud Hournai, said the rescuers were still pulling bodies from the underground shelter by early afternoon. He put the death toll at 20, saying women and children were among the casualties. "They are still pulling people from under the rubble," Hourani said. "The situation on the ground is disastrous." The Observatory said at least 46 civilians, including 15 children, have been killed since the offensive began, including the victims of Thursdays airstrike. The international aid organization CARE said humanitarian workers in the south are struggling to deliver basic needs to the population. One aid worker in an organization supported by CARE was killed in shelling on Tuesday while on duty. "Civilians are paying the price of another military offensive. What we have seen in Aleppo, northern rural Homs, and (Damascus suburb of eastern) Ghouta, is happening now in the south, where cities and towns are bombed daily, people are being uprooted and lack basic human necessities, such as water and shelter," said Wouter Schapp, CAREs Syria country director.