Syrian rebels say they have overrun an army garrison that defends the main southern border crossing with Jordan on Friday and vowed to press on to take control of the major transit route.
Fighters from the Free Syrian Army said on Friday that they captured the Um al-Mayathen post on the main Damascus-Jordan highway in heavy fighting overnight that ended a more than week-long siege.
Dozens died in the clashes, they added.
"It [the garrison] is a major defence and now we will lay siege to the border crossing and cut their [the government's] supply lines," Abu Omar, commander of the Lions of the Sunna Brigade, told the Reuters news agency by phone.
Confirming the development, the Syrian Observatory for Human Rights (SOHR) said: "Rebel fighters took control of the Um al-Mayathen military checkpoint ... in Deraa province in clashes with regime forces.
"Two fighters were killed and others wounded."
The UK-based watchdog group said there was no immediate word on any army casualties.
The army post is several miles from Syria's Nasib border crossing which, before the two-year-old civil war broke out, handled billions of dollars of trade between Gulf countries, Turkey and Europe.
Deraa has seen fierce fighting throughout the uprising against President Bashar al-Assad.
'Playing with fire'
The announcement comes amid reports that Jordan has tightened security along its border, as Syrian state media warned that the kingdom is "playing with fire" by allowing the US and other countries to train and arm rebels on its territory.
A Jordanian security official said on Thursday that the kingdom had tightened security along its 370km border with Syria, including the doubling of the number of soldiers in the last two days, though he declined to disclose the size of the force.
The Syrian warning, coinciding with significant rebel advances near the border, plays into Jordanian fears that its larger neighbour might try to retaliate for its support of the opposition fighters.
It followed statements from US and other Western and Arab officials that Jordan has been facilitating arms shipments and hosting training camps for Syrian rebels since last October.
A front-page editorial in the official daily al-Thawra accused Amman of adopting a policy of "ambiguity" by training the rebels while at the same time publicly insisting on a political solution to the Syrian crisis.
Jordan is "playing with fire", state radio said.
The rebels reportedly being trained in Jordan are mainly secular tribesmen from central and southern Syria who once served in the army and police.
The force is expected to fill a security vacuum by protecting the border with Jordan, assisting displaced Syrians and setting up a safe haven for refugees.
They are also envisioned as a counterbalance to al-Qaeda-linked groups that have proven to be among the most effective of the myriad rebel factions fighting Assad's forces on the ground.
In-depth coverage of escalating violence across Syria
"Jordan can't sit idle and watch al-Qaeda and other militants seizing control of its common border with Syria," Sameeh Maaytah, Jordanian information minister, said.
"It must take proactive steps to arrive at a state of equilibrium in the security structure on the border."
In another development on Friday, the United Nations Children's Fund, better known as UNICEF, gave warning that it would soon run out of money to cope with the vast influx of Syrian refugees into neighbouring countries.
By the end of the year, it estimates that there will be more than 1.2 million Syrian refugees in Jordan - which is equivalent to a fifth of its population.
The UN says it has only received a third of the money it needs to help them.