(Adds details, U.N. post pounded, monitoring group assessment)
By Baz Ratner
EIN ZIVAN, Golan Heights, Sept 1 (Reuters) - Heavy fighting between the Syrian army and Islamist rebels erupted on Monday on the Golan Heights, where the militants have wrested control of a key frontier crossing which had been operated by the United Nations.
It was not clear whether the forces loyal to Syrian President Bashar al-Assad had managed to retake control of the Quneitra crossing from the rebels of the al Qaeda-linked Nusra Front.
Persistent gunshots and explosions from mortar shells and other munitions could be heard on the Israeli-controlled side of the frontier and combatants could be clearly seen targeting each other with their weapons.
At least one tank belonging to the Syrian army was also involved and some rebels could be seen a few metres (yards) away from the frontier fence.
A large Syrian flag that had been flying for days between the Quneitra crossing and the abandoned town was taken down and a United Nations position in the area, thought to be unmanned, was pounded with mortar shells.
The Syrian Observatory for Human Rights, which monitors violence in the Syrian civil war, said the Nusra Front and allied fighters were battling government forces near the Quneitra crossing and in the nearby village of al-Hmaydieh.
The Observatory, which says it gathers information from all sides in the conflict, said there were confirmed casualties on both sides.
Speaking to Reuters, Observatory founder Rami Abdelrahman said the Nusra Front's aim appeared to be "to end once and for all the regime's presence in the area and it also appears that the goal is to expel the international observers".
DRONE SHOT DOWN
On Sunday, Israel's military said it had shot down a drone that flew from Syria into Israeli-controlled airspace over the Golan.
It was not immediately clear who had dispatched the unmanned aircraft or the nature of its mission in an area where fighting from Syria's civil war has occasionally spilled over into Israeli-held territory.
In a statement, the military said the drone was downed by a Patriot missile near the Quneitra crossing between the Israeli-held Golan Heights and Syria.
A contingent of 44 U.N. peacekeepers from Fiji was detained on the Syrian side of the Golan by Islamist militants on Thursday. The head of the Fijian army said on Sunday negotiations for their release were underway.
More than 70 Philippine troops trapped by Islamists in a different area on the Syrian side of the frontier have been moved to safety, the United Nations and authorities in Manila said.
Israel captured the Golan Heights in the 1967 Middle East war and beefed up defences in the area after the outbreak of the Syrian civil war more than three years ago.
A U.N. peacekeeping force has monitored the disengagement zone on the plateau between Israel and Syria since 1974, following the 1973 Arab-Israeli war. (Additional reporting by Tom Perry in Beirut, Writing by Ori Lewis in Jerusalem; Editing by Gareth Jones)