Near Damascus, a food aid convoy entered the town of Daraya in the first such delivery since the start of a regime siege in 2012.
IS has been under pressure on various fronts in Syria and Iraq, where it established its self-declared "caliphate" in 2014, and the extremists lost control Friday of a vital supply artery when Arab-Kurdish forces surrounded a key jihadist-held town.
"The Syrian Democratic Forces (SDF) cut off the last road from Manbij to the Turkish border," the Syrian Observatory for Human Rights said.
IS still controls territory along the Turkish border with secondary roads to the frontier but these are more dangerous and difficult to access, Observatory head Rami Abdel Rahman said.
This week the SDF, backed by air strikes by a US-led coalition, cut the road north out of Manbij to the IS-held border town of Jarabulus, which the jihadists had used as a transit point for fighters, money and weapons.
The SDF also blocked the road south out of Manbij heading to IS's de facto capital of Raqa.
"For the jihadists to reach the Turkish border from Raqa, they now have to take a route that is more dangerous because of regime troops nearby and Russian air strikes," Abdel Rahman said.
Russia launched air strikes in support of President Bashar al-Assad's regime in Syria in September.
Thousands of residents have fled Manbij -- held by IS since 2014 -- but jihadists who evacuated their families stayed to defend the town, the Observatory said.
About 20,000 people are still living in the town, which had a pre-war population of about 120,000 -- mostly Arabs, but about a quarter Syrian Kurds.
Food reaches rebel enclave
Syria's war has killed more than 280,000 people and displaced millions since it started in 2011 with the brutal repression of anti-government protests.
The UN says a total of 592,000 people live under siege in Syria -- most surrounded by government forces -- and another four million in hard-to-reach areas.
The Red Crescent said that vital food aid had reached the rebel-held town of Daraya, near Damascus, on Thursday for the first time in four years.
"Nine lorries are currently being unloaded in Daraya. They are carrying food aid, including dry goods and flour, non-food aid as well as medical aid," said Tamam Mehrez, operations director of the Syrian Red Crescent.
Last month, the SDF launched attacks on two fronts from the north of Raqa province towards Manbij and in direction of the IS-held town of Tabqa on the same vital supply line further south.
Regime troops backed by Russian air strikes have also pushed an offensive to the southwest of Tabqa.
Moscow and Washington -- despite backing different sides in Syria's five-year conflict -- have both focused efforts on fighting the jihadist group.