Syrian activists say rebels, including members of an al-Qaida-linked group, captured a government military post Saturday on the border with Jordan after four days of heavy fighting.
The Britain-based Syrian Observatory for Human Rights says 26 soldiers were killed in the battle at Deraa, and at least seven foreign fighters aligned with rebels.
The fall of the border post, which once served as a customs office, comes as United Nations weapons experts in Damascus prepared to investigate seven sites across the country for evidence of alleged chemical attacks.
It also follows a vote by the U.N. Security Council approving a resolution requiring Syria to eliminate chemical weapons.
The vote late Friday ended weeks of intense diplomacy between Russia and the United States, triggered by an August 21 sarin nerve gas attack that killed hundreds in a Damascus suburb.
The U.S.-Russia deal averted punitive U.S. military action against the government of President Bashar al-Assad, which Washington blamed for the attack.
U.S. President Barack Obama called Friday's Security Council resolution a "huge victory" for the international community. He said it is a "legally binding" and "verifiable" initiative that threatens consequences if Syria does not adhere to conditions.
The resolution calls for inspections within 30 days at all chemical weapons sites declared by Syria's government. International experts are supposed to begin inspecting Syria's chemical arsenal by Tuesday.
The resolution also requires Assad to give up his chemical weapons by mid-2014.