IS has come under growing pressure on various fronts in Syria and Iraq, where it established its self-declared "caliphate" in 2014.
The extremists lost control Friday of a vital supply artery when Arab-Kurdish forces completely surrounded a key jihadist-held town.
"The Syrian Democratic Forces (SDF) cut off the last road from Manbij to the Turkish border," said the Syrian Observatory for Human Rights, a Britain-based monitoring group.
Manbij lies at the heart of the last stretch of territory along Turkey's border still under IS control, and was a key point on the jihadists' supply line from Turkey.
Other secondary roads to the frontier are more dangerous and difficult to access, Observatory head Rami Abdel Rahman said.
The US envoy to the anti-IS coalition backing the SDF, Brett McGurk, confirmed the road had been severed.
"ISIL terrorists now completely surrounded with no way out," he wrote on Twitter, using another acronym for IS.
"Manbij is where we believe the Paris attackers, the Brussels attackers, they all kind of pulsed through this area," McGurk said, "from Raqa up to Manbij and then out to the capitals where they had organised their attack."
This week the SDF, backed by coalition air strikes, 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.
- Food reaches rebel enclaves -
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.
Last month, the SDF attacked on two fronts from the north of Raqa province towards Manbij and in the direction of the IS-held town of Tabqa on the same vital supply line further south.
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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.
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 United Nations 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 Syrian Arab Red Crescent and the UN said aid was delivered to Douma on Friday. The SARC said 39 trucks took food and non-food items including medicines into the besieged town.
According to the Observatory, it was the first time that the United Nations delivered food aid to Douma since autumn 2013.
Late on Thursday, another food aid convoy approved by the regime entered the rebel-held town of Daraya near Damascus in the first such delivery since the start of a regime siege in 2012.
- Barrel bombs prevent handout -
But the Observatory and a resident said the regime dropped barrel bombs -- crude unguided explosive devices -- on Daraya on Friday, preventing residents from receiving the desperately needed food.
"Aid received by the council has not been distributed yet because of the intensity of the raids," local council member Shadi Matar said.
French Foreign Minister Jean-Marc Ayrault voiced outrage over the barrel-bombing.
He accused Damascus of "extraordinary duplicity", saying the regime had finally granted access for aid after heavy international pressure "and then the bombing restarted."
The Observatory and local council estimate that 8,000 people live in Daraya, one of the first towns in Syria to erupt in anti-government demonstrations in 2012 and one of the first to come under regime siege the same year.
But the United Nations speaks of 4,000 besieged residents, angering inhabitants who say the food delivered is not nearly enough.
A previous UN aid convoy reached Daraya on June 1 but contained no food.
The town is just a 15-minute drive from central Damascus and is even closer to the regime's Mazzeh air base, which hosts the feared air force intelligence services and their notorious prison.
The UN's humanitarian agency said on Friday it was still awaiting permission from Damascus to deliver aid to two more of Syria's besieged areas, Al-Waer in Homs province and Zabadani in rural Damascus.
UN-backed peace talks on ending the war have stalled after the opposition walked out of negotiations in April over lack of humanitarian access.
A nationwide ceasefire between the regime and non-jihadist rebels -- brokered by Russia and the US -- is in tatters.