Syrian rebels repelled an advance by government forces near a strategic highway in northern Syria on Tuesday, April 16in fierce battles expected to inflict heavy casualties on both sides, opposition activists said, according to Reuters.

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.