Fighters try to reclaim land lost near Sheikh Maskin after major offensive by government forces and Russian air power.

Syrian rebels say they have pushed back a major government offensive near the southern city of Sheikh Maskin as the battle intensifies for control of key supply routes in the nearly five-year-old civil war.

Fighting on Thursday came as troops loyal to President Bashar al-Assad – backed by Russian air power – attempt to re-establish regime control of Deraa province.

Holding Sheikh Maskin is crucial to both the government and the rebels as it is a key supply route connecting the opposition in the south to territories in the suburbs of Damascus.

A government victory would force rebel factions – including al-Nusra Front and Western-backed Free Syrian Army groups – to withdraw from the province completely.

Up until the government advance, relations between the opposition groups had been marked by frequent infighting and a lack of coordination. That has changed however with the onslaught.

Al Jazeera’s Hashem Ahelbarra, reporting from Adana in southern Turkey, said defeat for the rebels would put them in a difficult position when negotiations between the sides begin in January.

READ MORE: Syrian troops and Russian jets push for strategic town

“The opposition has called on the rebels to set their differences aside and defend Sheikh Maskin. A defeat at this particular moment could undermine the chances of the rebels pushing for more concessions from the government in upcoming talks that will be held in Geneva,” Ahelbarra said.

Syria’s civil war will mark its fifth anniversary in March. The conflict began when an initially peaceful revolt against Assad was put down with deadly force.

The first protests against Assad’s rule started in Deraa province in March 2011 before growing into a full-blown armed conflict that has killed more than 250,000 people, according to UN statistics.

Meanwhile, the Syrian Observatory for Human Rights, a monitoring group, said on Thursday that more than 55,000 people, including almost 30,000 civilians, have been killed in Syria in 2015 alone.