BEIRUT (Reuters) - Syrian rebels repelled an advance by government forces near a strategic highway in northern Syria on Tuesday, in fierce battles expected to inflict heavy casualties on both sides, opposition activists said.
The two sides are struggling for control of a highway that serves as the main route into Aleppo, Syria's largest city, after President Bashar al-Assad's forces broke through a six-month rebel blockade of two bases near the road.
Rebels are determined to re-establish the blockade of the bases, located outside the town of Maarat al-Nuaman in Idlib province, because a government advance could upset the balance of power in the heart of the rebel-held north.
No side now fully controls the highway.
If the army were able to capture the main artery into Aleppo it could bolster its fragile supply lines and hit harder at the insurgents, who despite their gains remain vulnerable to daily air strikes on towns under their control.
Rebels maintained the blockade operation despite a high cost in lives and weaponry to keep the army bottled up in the Wadi al-Deif and Hamidiya bases.
They had been unable to storm the bases and were criticized by some in the opposition for depleting their own forces due to infighting and the deployment of many units to other towns.
On Sunday, government forces outflanked the rebels and broke through, state media said.
But rebels were able to drive Assad's forces out of the village of al-Tah by Tuesday, activists said on a local opposition Facebook page, pushing them back to the town of Babolin, further south.
"The rebels were able to advance their counter-offensive. There will be a heavy casualty toll of dead and wounded, but we cannot offer precise figures yet," Rami Abdelrahman, head of the Syrian Observatory for Human Rights, told Reuters by telephone.
Abdelrahman, who manages a network of activists across Syria from his Britain-based group, said it was not yet clear who was gaining the upper hand in the past months of clashes.
Two years into the uprising against Assad, government forces are fighting hard to keep control of cities. Many rural areas and provincial towns have fallen to the rebels. Aleppo is caught in a stalemate between the rival forces.
Rebels have advanced in northern Syria, near Turkey, and the southern province of Deraa near Jordan.
But government forces have kept the rebels out of central Damascus and hold more than half of Homs, Syria's third largest city, which connects the capital to Assad's Alawite heartland near the Mediterranean coast.
Syria's uprising began as a protest movement against four decades of Assad family rule but has degenerated into an increasingly sectarian conflict that has killed at least 70,000.
Sunni Muslim rebels form the backbone of the insurgency, while minorities like the Alawites, from an offshoot of Shi'ite Islam, have largely fought with Assad, whose family has ruled the major Arab state for 43 years.
Rebel attacks had forced the army to relinquish many bases in northern Syria and most roads around Aleppo and Idlib, leaving remaining government-controlled areas in the north to rely on airlifts for food and weapons.
The army has yet to score a decisive victory in the area but their recent advance battered the rebels. Abdelrahman counted more than 50 fighters dead or missing from the battle on Sunday.
Activists in Maarat al-Nuaman, which has suffered daily air strikes and artillery barrages due to the blockade, accused rebels of causing their own setback by not resolving rising tensions between Islamist groups and the more politically moderate - but less militarily effective - rebel units.
According to Abdelrahman, many of the main fighting units previously based in the area had moved to Raqqa, Ras al-Ain, and Hassakah, towns in the northeast that rebels led by Islamist brigades recently seized.
As rebel units have moved to more successful battlefronts, the army has been pushing hard on areas where they had made gains around Aleppo, Idlib and Damascus. (Editing by Mark Heinrich)