Rebels including Al-Qaeda's local affiliate seized the Syrian regime's largest remaining military base in northwestern Idlib province on Tuesday, a monitor said.
The Syrian Observatory for Human Rights also reported that around 170 jihadists were killed in US-led air strikes in the past 48 hours in support of Kurdish forces in northeast Syria.
The loss of Al-Mastumah base leaves only a few positions in regime hands in Idlib, a region that borders Turkey and neighbours the government stronghold of Latakia province on the Mediterranean coast.
"All regime forces have withdrawn from Al-Mastumah, the largest regime base in Idlib, leaving it completely in the hands of opposition fighters," said Rami Abdel Rahman, head of the Observatory.
The capture of the base followed a huge blast inside and heavy clashes, though the cause of the explosion was not immediately clear.
Al-Qaeda affiliate Al-Nusra Front, a key player in the Army of Conquest grouping that has seized much of Idlib in recent weeks, announced Al-Mastumah's capture online.
"With the help of God, Al-Mastumah was completely liberated after Al-Nusra Front stormed it from the south," Al-Nusra said on Twitter.
Abdel Rahman said regime troops withdrawing from the base were leaving towards Ariha, now one of the last government bastions in Idlib.
State television said the army was "taking defensive positions" outside of Ariha, implying that army units had withdrawn from Al-Mastumah itself.
The base's capture comes after the rebel grouping, known in Arabic as Jaish al-Fatah, seized Idlib's provincial capital on March 28 and the nearby city of Jisr al-Shughur last month.
The Army of Conquest had since edged closer to Al-Mastumah, where regime forces fled after withdrawing from Idlib city.
There was no immediate toll in the capture of the military base.
- 170 jihadists dead -
In Hasakeh province of northeast Syria, around 170 jihadists were killed in 48 hours, mostly in "very intense air strikes" by the US-led coalition, the Observatory said.
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The US military said the alliance's warplanes carried out seven strikes near Hasakeh.
"In northern Syria, Daesh continues to cede military capacity, fighters and terrain," US Brigadier General Thomas Weidley said, using the Arabic acronym for the Islamic State.
Elsewhere on Tuesday, IS attacked a village in the Druze heartland of southern Syria, killing six people before pro-regime gunmen expelled them.
"IS killed five fighters from the National Defence Forces and one woman in their attack on the Druze village of Al-Haqef in Sweida province," Abdel Rahman said.
State news agency SANA said the army and NDF "foiled an infiltration attempt by terrorists from IS on the villages of Al-Qasr and Al-Haqef".
To the north, fighting between IS militants and government forces continued outside the historic town of Palmyra on Tuesday.
"There are clashes at the western entrance of Palmyra this morning," provincial governor Talal Barazi told AFP.
On Sunday, regime forces pushed IS out of northern neighbourhoods of Palmyra, which it had held for less than 24 hours.
- Palmyra under threat -
Barazi said he visited Palmyra on Monday, "going through 60 percent of the city on foot," including the vegetable market and museum.
He expected the situation to return to "normal" within a week.
At least 40 rockets struck Palmyra on Sunday, he said, but government forces maintained control over key points, including the Islamic citadel overlooking the city.
Palmyra, whose archeological treasures are renowned worldwide, lies along major highways linking Damascus and Homs to the west, with the Syrian desert and the Iraqi border to the east.
The Observatory reported fresh clashes north of the city and that the government had brought in reinforcements.
To the east, an air strike on a village in Deir Ezzor province killed at least eight civilians, including three children, the Observatory said.
Syria's conflict began with anti-government protests in March 2011 but has evolved into a multi-front war that has killed more than 220,000 people.