Hundreds of troops in a large military convoy with Russian and Syrian flags advanced on Syria's Nassib border crossing with Jordan, witnesses said.
The Observatory said that with the capture of Saida, Syrian troops are now about 4 miles from the Naseeb border crossing with Jordan, one of the main goals of the offensive.
Syrian army troops came close to the Jordanian border on Thursday as the opposition and Russian Federation agreed to resume suspended talks on a deal to end fighting that has sparked one of the fastest displacements in the course of the conflict.
The UN refugee agency says around 60,000 Syrians have gathered near the border.
The crossing, a key trade route, was held by opposition groups almost for three years.
The United Nations coordination office for humanitarian affairs said they had died in areas close to the Jordanian border due to "scorpion bites, dehydration and diseases transmitted through contaminated water".
The Syrian government forces retook a security checkpoint along the Jordanian border for the first time in more than three years.
They said Russian Federation had insisted opposition factions hand over their heavy weapons in one go, while rebels wanted to do so in several phases.
A regime air strike on the rebel-held half of the provincial capital of Daraa also killed one civilian, the Observatory said.
The de-escalation zone in the southwest was agreed by Russia, Jordan and the U.S. in June 2017.
Osama Al Homsi, 26, said he was hesitant to return to his hometown of Jeeza in southeastern Daraa after the deal.
Regime forces bombarded southern Syria on Sunday, as the evacuation of rebel fighters under a ceasefire deal for the region was postponed, a monitor and an opposition official said.
Another displaced Syrian, Umm Zaid, a mother of five who had lost her husband in the latest offensive, said she would break through the fence if necessary, speaking to Reuters from the border area by phone.
With no sign of intervention yet by his foreign foes, government forces seem set for another big victory in the war after crushing the last remaining rebel bastions near Damascus and Homs.
Calm reigned over the region on Saturday as the two sides finalised the ceasefire deal, according to the Syrian Observatory for Human Rights, a Britain-based monitoring group.
The UN refugee agency (UNHCR) on Thursday warned that 320,000 people have been displaced by increased hostilities in southwest Syria while another 750,000 risk being caught in the crossfire.
Still, the border has been closed since the rebels captured it in 2015.
The military's next target in the southern offensive appears to be the parts of nearby Quneitra province in rebel hands at the Golan frontier.
Hezbollah has sent thousands of fighters to Syria since 2013 to help Assad's forces, tipping the balance of power in his favor in several areas, including along the border with Lebanon.