Syrian activists say Nusra Front, an al-Qaeda-linked group battling the Syrian regime, is in full control of a military residential compound in Deraa city.
The Sunni group reportedly set off two suicide-car bombs on Sunday, targeting the sleeping quarters of soldiers and officers from the Syrian army.
With the latest gains, Nusra Front has come a step closer to the strategic town of Azraa, one of the last defence lines of Bashar al-Assad's regime before Damascus province.
In a separate development, Syrian government forces repelled an attack by fighters from the Islamic State of Iraq and the Levant (ISIL) group on a military airport in the eastern province of Deir Az Zor, according to a monitoring group.
"Troops and pro-regime militia stopped the attack that Islamic State launched on the Deir Az Zor military airport," the UK-based Syrian Observatory for Human Rights said, adding that both sides suffered heavy losses in the fighting.
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More than 100 fighters and about 60 troops were killed since Wednesday when ISIL launched the operation to try to take the airport, the Syrian Observatory said.
State news agency SANA said the army had managed to repel the ISIL fighters, but provided no further details.
Elsewhere in Syria, rebels blew up a tunnel near an ancient mosque in the northern city of Aleppo, the Syrian Observatory said.
State television also reported the explosion, saying that the rebels blew up the Sultaniyeh Mosque itself.
The Syrian Observatory added that the army secured a fresh advance in Aleppo province, killing at least 24 rebels.
"The army ... took the Breij area northeast of Aleppo city," Rami Abdel Rahman, director of the Syrian Observatory said.
The soldiers' advance meant the army was closing in on rebels in the east of Aleppo city.
"There is a very real threat that the opposition's supply route will be cut off," Abdel Rahman said.
The Syrian Observatory said it could not confirm the state TV's claim that the Sultaniyeh Mosque had been damaged.
Source: Al Jazeera And AFP