Fighters affiliated to the so-called Islamic State group are reported to have tightened their grip on a Syrian government supply route to Aleppo as the army battled to retake the road as part of its campaign to seize the city.
As the Syrian government accepted a US-Russian plan for a "cessation of hostilities" between the government and rebels due to take effect on Saturday, heavy Russian air strikes were also said to be targeting one of the last roads into opposition-held parts of Aleppo.
The plan, announced by the United States and Russia yesterday, is the result of intensive diplomacy to end the five-year-long war.
Rebels say the exclusion of IS and the al Qaeda-linked Nusra Front will give the government a pretext to keep attacking them because its fighters are widely spread in opposition-held areas.
The Syrian government, backed by Russian air strikes since September, said it would coordinate with Russia to define which groups and areas would be included in what it called a "halt to combat operations".
The government also warned that continued foreign support for the rebels could wreck the agreement.
The Russian intervention has turned the momentum President Bashar al-Assad's way in a conflict that has splintered Syria and mostly reduced his control to the big cities of the west and the coast.
Forces loyal to Mr Assad, backed by ground forces including Lebanon's Hezbollah and Iranian Revolutionary Guards, is making significant advances, including near the city of Aleppo which is split between rebel and government-control.
The IS assault has targeted a desert road which the government has been forced to use to reach Aleppo because insurgents still control the main highway further west.
The Syrian Observatory for Human Rights, which reports on the war using a network of sources on the ground, said IS fighters had seized the village of Khanaser on the road, which remained closed for a second day.
A Syrian military source said army operations were continuing to repel the attack.
IS, which controls swathes of eastern and central Syria, differs from rebels fighting Mr Assad in western Syria because its priority is expanding its own "caliphate" rather than reforming Syria through Mr Assad's removal from power.
The group has escalated attacks on government targets in recent days.
On Sunday, it staged some of the deadliest suicide bomb attacks of the war, killing around 150 people in government-controlled Damascus and Homs.
A US-Russian statement said the two countries and others would work together to delineate the territory held by IS, Nusra Front, and other militant groups excluded from the truce.
In Geneva, UN spokesman Ahmad Fawzi said: "This is a cessation of hostilities that we hope will take force very quickly and hope provide breathing space for intra-Syrian talks to resume."
Mr Fawzi said there were plans for additional aid deliveries to opposition-held areas blockaded by government forces near Damascus, including the Eastern Ghouta.