Syrian regime forces pounded opposition-held areas of the southwest with artillery yesterday, in a steady escalation by Bashar al-Assad who has vowed to win back the area bordering Jordan and the Israeli-occupied Golan Heights. The Syrian Observatory for Human Rights said 12,500 civilians had fled the town of Busra al-Hariri and nearby areas of Deraa province in the last two days. Opposition officials gave conflicting assessments of the scale of displacement.

A major government offensive in the southwest risks further escalating the seven-year-old war. The United States has warned it would take "firm and appropriate measures" in response to government violations of a "de-escalation" deal it brokered with Russia last year to contain the conflict in the southwest.

The conflict has pivoted towards the southwest since Assad, with critical help from his Russian and Iran-backed allies, crushed the last remaining pockets of opposition-held territory near Damascus and the city of Homs. Although he has vowed to win back the area, a military campaign to recover it is set to be complicated by the interests of both U.S.-allied Jordan and Israel, which is deeply alarmed by Iran's role in Syria. As more civilian continue to flee the region amid heavy shelling, the United Nations was "concerned about reports of an escalation of violence in Deraa governorate", which was "endangering civilians and displacing hundreds of families", Jens Laerke, the U.N. humanitarian spokesman in Geneva, said.