Rebels and Al Qaeda's Syria affiliate clashed Tuesday with jihadists linked to the Islamic State group (ISIS) along Syria's border with the Israeli Golan Heights, a monitor and an opposition spokesman said.
Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, told AFP that rebel groups, as well as Al Qaeda affiliate Al-Nusra Front, fought ISIS-linked Jaish al-Jihad in the border town of Qahtaniya in the southern province of Quneitra province.
"These are key battles, because IS now has a presence in Syria's south and because they are close to the ceasefire line with the Golan," said Abdel Rahman.
"This is the first time these groups have clashed with each other," he added.
Abdel Rahman said 12 rebels, including Al Nusra fighters, were killed and Jaish al-Jihad lost seven fighters.
"Fifteen members of Jaish al-Jihad were also taken hostage," he said.
The clashes took place as Israel announced that two mortar rounds fired from Syria struck northern parts of the Golan Heights, without causing casualties. The IDF later stated that the mortar fire was the result of errant "spillover" from the conflict, and not a deliberate attack.
Jaish al-Islam is a relatively new group but is suspected to have very close ties to ISIS, according to Romain Caillet, a French expert on jihadists.
Essam al-Rayes, spokesperson for the Southern Front coalition which is fighting Jaish al-Islam, said the ISIS-linked group had a presence in and around the town of Qahtaniya.
Speaking by phone from Jordan, Rayes said tensions among opposition fighters in the town rose after it was suspected that Jaish al-Jihad had pledged allegiance to ISIS.
The battles erupted on Monday after Jaish al-Jihad ambushed a Southern Front convoy and killed six rebels, Rayes added.
"Al Nusra Front then joined the fight, because Daesh is our common enemy," Rayes said, using the Arabic acronym for ISIS.
Caillet said two Saudi commanders from Al Nusra's southern battalions were killed on Monday night.
The Southern Front had been planning a large offensive on regime-controlled areas north of Qahtaniya, but the latest clashes delayed their assault, Rayes told AFP.
Last month, rebel groups in southern Syria, including the Southern Front, said they would not cooperate with Al Nusra in future battles, after a dispute over a captured border crossing.
Syria's conflict began in 2011 with peaceful protests, but has since become a complex civil war which has killed more than 220,000 people.