Rebels fighting President Bashar al-Assad's regime have seized a 25-kilometre (15 miles) strip of land stretching from the Jordan border to the Israeli-occupied Golan Heights, a watchdog said on Sunday.
"Fighters loyal to Al-Nusra Front, Al-Yarmuk Brigade, Al-Mutaz Billah Brigade and others took control of Al-Rai military checkpoint," east of Sahem al-Golan town in the southern province of Daraa, said the Syrian Observatory for Human Rights.
"The fighters seized the site after regime forces retreated. The 25-kilometre area located between the towns of Muzrib (near the Jordanian border) and Abdin (in the Golan) is now out of regime control."
The Britain-based Observatory said the rebels seized in the past few days several army checkpoints in the area and captured weapons and vehicles.
The rebels on Saturday seized a key air base Daraa after two weeks of fierce battles with loyalist troops, it added.
The report came as Israel's new Defence Minister Moshe Yaalon on Sunday vowed an "immediate" answer to all Syrian gunfire onto the Golan Heights.
Yaalon issued the warning shortly after Israeli troops on the strategic plateau shot at an Syrian army post after coming under fire for the second time in 12 hours, according to the Israeli army.
"We see the Syrian regime as responsible for every breach of sovereignty. We shall not allow the Syrian army or any other body to violate Israeli sovereignty firing into our territory," Yaalon said in a statement.
It was not immediately clear whether the shooting was from the Syrian army or from rebel forces in the area.
The rebel advances came days after insurgents seized a border crossing on the frontier with Jordan, said the Observatory.
A security source in Damascus told AFP this week that Jordan was allowing jihadist fighters and arms bought by Saudi Arabia from Croatia to be smuggled into Syria.
The source said that around 2,500 trained and heavily equipped rebels have entered Daraa in recent weeks, following reports that American instructors were training rebels in neighbouring Jordan.
Jordanian Information Minister and government spokesman Samih Maaytah said earlier this month that his country "rejects interfering in Syrian affairs."
"The Jordanian army is exerting a lot of efforts to control the border and prevent infiltrations," he said.
Louay Moqdad, a spokesman and coordinator for opposition forces, acknowledged that several Arab and Western nations had started training rebels forces, but declined to provide further details.
Earlier this month, the German magazine Der Spiegel reported rebels were being trained in Jordan by American specialists, a claim US officials have refused to comment on.
The United Nations estimates that violence across Syria has killed at least 70,000 people since the conflict erupted in March 2011.
The Observatory said that 108 people were killed in violence across Syria on Saturday.