The Syrian Observatory for Human Rights said troops still needed to clear the jihadists from hills overlooking the key highway before it could be fully secured.
The road through the town of Khanasser is the sole link between government-held areas in and around Aleppo and those in the rest of the country.
State news agency SANA hailed its recapture just two days after its fall to the jihadists.
The army's counteroffensive was backed by Russian air strikes which killed 20 IS fighters, said the Observatory, a Britain-based monitoring group.
It is not the first time that the government's supply route to Aleppo has been cut. Rebels severed it in 2013 and IS also did so last year before being pushed back.
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The new fighting for the road comes with the army on the offensive north and west of Aleppo, where rebel forces in the east of the city are almost completely surrounded.
That offensive is supposed to come to an end when a Russian and US-brokered ceasefire goes into effect at 2200 GMT on Friday.
But the war against IS will go on as the group is specifically excluded from the planned truce, as is Al-Qaeda's Syrian affiliate Al-Nusra Front.
Turkey has pushed for the exclusion too of the main Syrian Kurdish militia, the People's Protection Units (YPG), which has also seized territory from rebels northwest of Aleppo in recent weeks.
Turkish artillery shelled YPG-held territory in Aleppo province throughout Wednesday night, the Observatory said, without immediately reporting any casualties.
The renewed bombardment came after Turkish President Recep Tayyip Erdogan on Wednesday again lashed out at Western policy towards the YPG, charging that the militia were a "terror group" just like IS and Al-Qaeda.
To the anger of Ankara, which regards the YPG as a branch of outlawed Turkish rebel group the Kurdistan Workers' Party (PKK), Washington has worked closely with what it regards as the most effective fighting force against IS in Syria.