Beirut (AFP) - Al-Qaeda's Syrian affiliate and other Islamist fighters made gains on Friday in a joint offensive on the last major regime bastion in the northwest province of Idlib, a monitor said.
The assault, which began on Thursday, has seen Al-Nusra Front and allied groups seize three checkpoints around the large town of Jisr al-Shughur, the Syrian Observatory for Human Rights said.
The town is strategically located near the border with Turkey and along a key route leading west to the regime's stronghold of Latakia province.
"There are very fierce clashes ongoing since the early morning, and intense aerial bombing. The regime has conducted 34 air strikes in the area since Friday morning," Observatory head Rami Abdel Rahman told AFP.
He said Al-Nusra Front had launched a wave of suicide attacks on the town's outskirts and sent 15 fighters wearing suicide belts into Jisr al-Shughur itself.
Since the fighting erupted on Thursday, 30 regime troops and 43 rebels -- including 13 Chechen fighters -- have been killed, Abdel Rahman said.
The Islamist coalition was battling pro-government forces for three other checkpoints around the town, Abdel Rahman added.
On its official Twitter and Facebook accounts, Al-Nusra Front posted pictures of explosives and destroyed buildings, saying it would "purify Jisr al-Shughur" of President Bashar al-Assad's forces.
The local branch of the Syrian Revolution General Commission activist group said rebels began attacking from the north, leaving regime forces "floundering".
The Syrian regime made Jisr al-Shughur its de facto provincial capital after the same rebel coalition calling itself "The Army of Conquest" overran Idlib city last month.
The regime still controls areas to the east, including the town of Ariha, a military base in Al-Mastumah and an air base at Abu Duhur.
- Regime warplane downed -
But seizing Jisr al-Shughur would cut regime access to a major highway linking Idlib to Latakia -- Assad's home province -- farther west.
If opposition groups overrun it, it would enable them to launch additional attacks on Latakia, said Charles Lister, visiting fellow at the Brookings Doha Centre think tank.
"This would be very dangerous for the regime," he said.
Al-Nusra Front and its allies already control much of Idlib province.
Elsewhere, the extremist Islamic State group shot down a regime aircraft near a key military airport in southern Syria, with pro-IS Twitter accounts saying the jihadists had captured the pilot.
The plane went down east of Khalkhalah airport, the only air base in Sweida province, a stronghold of the Druze minority that has largely avoided the bloodshed of Syria's war, the Observatory said.
Abdel Rahman told AFP the fate of any crew members remains unknown.
State television, citing a military official, said an aircraft "crashed due to technical problems while completing a training exercise" near Khalkhalah, and that the search for the pilot was ongoing.
In its first major attack in Sweida, IS tried to storm the airport on April 11, but loyalist forces beat them back.
Khalkhalah lies along a major highway between Damascus and the regime-held city of Sweida.
In February, a Jordanian warplane conducting air strikes as part of the international anti-IS coalition crashed inside Syria.
IS captured pilot Maaz al-Kassasbeh and later burned him alive, releasing a grisly video on the Internet of the airman's death.
Syria's conflict has killed more than 220,000 people and displaced upwards of 11.2 million since it began in March 2011.