DAMASCUS — Syrian rebels have launched a fierce offensive against President Bashar Assad’s troops in four northern provinces, aiming to reverse a string of defeats, an NGO and rebels said Monday.

The offensive follows several victories for the regime, most recently in former rebel bastion Yabrud, in the Qalamun region along the border with Lebanon.

On Monday, the rebels and their jihadist ally Al-Nusra Front seized a key area on the border with Turkey, gaining full control of the Kasab frontier crossing, the Syrian Observatory for Human Rights said.

The takeover came even as the air force bombarded rebel positions in Kasab, in Latakia province, the heartland of Assad’s Alawite sect, a day after Turkey shot down a Syrian warplane in the area.

“It is clear that the opposition factions launched a fierce offensive in northern Syria after the battle for Qalamun,” said Observatory director Rami Abdel Rahman.

The assault has focused on Latakia, Idlib and Aleppo provinces, as well as in the north and west of Hama province.

“The rebel fighters have advanced in all these areas, while the regime has clearly retreated,” Abdel Rahman told AFP.

In Idlib province of northwest Syria, rebels have in recent days taken over a string of 15 army checkpoints.

In Aleppo, the rebels have advanced in the city and in parts of the countryside.

In Hama’s Morek, which lies on a key supply route linking the center of Syria to Aleppo, rebels have fought off repeated attempts by the army to break through their lines of defense, said the Observatory.

In Latakia province, opposition fighters launched a surprise offensive on Friday.

Rebels say the advances are connected.

“After the battle for the coast (Latakia) began, the army withdrew many of its fighters from Idlib to go fight there,” said Colonel Afif al-Suleimani, an officer who defected from the army and now heads Idlib’s rebel Military Council.

“This opened a gap here in Idlib and we took advantage of it, and went on the offensive,” he said.

“We are three provinces, but we are neighbors, and many of the factions have fighters on more than one front. So we help each other,” Suleimani told AFP via the Internet.

“There are many ties between us, and yes, we can speak of a joint offensive that has opened up.”

Now we only have one enemy

According to Ibrahim al-Idelbi, a rebel spokesman, a key factor behind opposition factions taking the initiative was the withdrawal around two weeks ago of the jihadist Islamic State of Iraq and the Levant (ISIL) from most of northern Syria.

A broad coalition of moderate and Islamist rebels has been fighting ISIL since January 3. Thousands of fighters on the two sides have been killed.

ISIL and Syria’s rebels were once allied in the fight against Assad, but the extremists’ quest for hegemony and its atrocities turned the rest of the opposition against them.

Most of ISIL’s fighters are now in Raqa, further east.

“ISIL’s exit from the area is a key factor in the advances. Now, rather than having two enemies, the rebels have one (Assad’s regime),” Idelbi told AFP.

“And the roads are now open for the rebels between the countrysides of Latakia, Idlib and northern Hama, opening up supply routes. There is a whole triangle, a section of Syria, that has been liberated,” he said.

The developments come a day after Turkey shot down a Syrian warplane that Ankara said had breached its airspace.

Damascus described the downing as a “flagrant act of aggression.”

Omar al-Jeblawi, an activist in Latakia, said: “The regime is very angry. They are using all their force — army and paramilitary — to try to stop the rebels, who in turn are trying to advance towards the sea.”

Kasab’s majority Armenian Christian residents have fled the village because of the violence, the Observatory’s Abdel Rahman said.

It is the last remaining Armenian village in the Middle East, dating back centuries, according to Syria expert Fabrice Balanche.

After three years of war, Syria’s government now controls only eight of the 19 official land border crossings with its neighbors Lebanon, Jordan, Iraq and Turkey.

Only one crossing into Turkey remains in government hands, but it is closed on the Turkish side.