BEIRUT - Syrian troops backed by Hezbollah fighters have killed at least 50 militants from the Islamic State and Al-Nusra Front groups near the border with Lebanon, a monitor said Saturday.The clashes raged through the night and into the morning on Saturday in the border region of Qalamun, said the Syrian Observatory for Human Rights. Among the dead were at least seven pro-regime fighters, including government soldiers and members of Hezbollah, the Lebanese Shiite militant group that backs Syria’s President Bashar al-Assad.Regime forces recaptured most of the Qalamun region in April, with many rebel fighters withdrawing from the strategic area or slipping across the border in Lebanon. But pockets of opposition fighters, including militants, have remained in the mountainous region. Though the Islamic State and Al-Nusra Front both have roots in Al-Qaeda, IS has formally broken with the group, while Nusra is its official branch in Syria.Despite ideological similarities, the two groups are opposed and in conflict with each other in other parts of Syria, particularly in the north. But in Qalamun, their fighters battled the regime and Hezbollah forces alongside each other, with support from some smaller Militant rebel groups, the Observatory said.Moreover, a local chief of Al-Qaeda’s Syrian affiliate Al-Nusra Front has died in an apparent assassination after a bomb went off in his car, a monitor said Saturday. “Nusra’s emir in Idlib (province), Yaacub al-Omar, was killed overnight when a bomb went off in his car near his house in the Khan al-Subul area,” the Syria Observatory for Human Rights said. Two of his sons were wounded in the blast, the Britain-based group added. Omar, a Syrian in his forties, took over as local chief in April after his predecessor, Abu Mohammed al-Ansari, was killed by the militant Islamic State group.Despite both having their roots in Al-Qaeda, Nusra and the Islamic State — formerly known as the Islamic State of Iraq and the Levant — have been in conflict since the beginning of 2014. Both groups oppose the Syrian regime, but Nusra joined moderate and Militant rebels to combat IS in a widespread backlash against its extremist ideology and abuses of civilians and rival rebels. However, in recent weeks, Nusra has also been fighting some of those erstwhile allies.That has further complicated the Syrian conflict, which began as an uprising against President Bashar al-Assad’s government, but has degenerated into a complex war involving multiple fronts and foreign fighters on both sides. The fighting between Nusra and other rebels began in early July, with a particularly bloody battle breaking out in the Jisr al-Shugur region of Idlib a week ago. Fighting there left dozens dead on both sides, according to the Observatory. Rebels in the region have accused Nusra of trying to push them out of Idlib so the group can create an “Islamic emirate” to rival the “Islamic caliphate” that IS has declared in parts of Syria and Iraq under its control.