BEIRUT (Reuters) – Islamic State fighters were reported to have tightened their grip on a Syrian government supply route to Aleppo on Tuesday as the army battled to retake the road, which is important to its campaign to retake the city.

As Damascus accepted a U.S.-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 on Monday is the result of intensive diplomacy to end the five-year-long war. But rebels say the exclusion of Islamic State 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”. Damascus 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.

Damascus, 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 Islamic State 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 the war using a network of sources on the ground, said the road remained cut for a second day. A Syrian military source told Reuters army operations were continuing to repel the attack.

Observatory Director Rami Abdulrahman told Reuters: “The clashes are ongoing, the regime recovered four of seven (lost positions). It is still cut.” It later reported IS had seized control of the village of Khanaser on the road.

Islamic State, which controls swathes of eastern and central Syria, differs from rebels fighting Assad in western Syria because its priority is expanding its own “caliphate” rather than reforming Syria through 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 U.S.-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, U.N. 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.”

RIGHT TO RESPOND

Damascus stressed the importance of sealing the borders and halting foreign support for armed groups and “preventing these organizations from strengthening their capabilities or changing their positions, in order to avoid what may lead to wrecking this agreement”.

The Syrian military reserved the right to “respond to any breach by these groups against Syrian citizens or against its armed forces”, a government statement added.

The main, Saudi-backed Syrian opposition body said late on Monday it “consented to” the international efforts, but said acceptance of a truce was conditional on an end to blockades of rebel-held areas, free access for humanitarian aid, a release of detainees, and a halt to air strikes against civilians.

The opposition High Negotiations Committee also said it did not expect Assad, Russia, or Iran to cease hostilities.

The powerful Kurdish YPG militia, which is currently fighting both Islamic State and rebels near Aleppo, is “seriously examining” the U.S.-Russian plan to decide whether to take part, a YPG official told Reuters. “There is so far no decision,” said the official, declining to be identified because he is not an official YPG spokesman.

The YPG, an ally of the United States in the fight against Islamic State in Syria, has recently received Russian air support during an offensive against rebels near Aleppo.

Britain said on Tuesday it had seen disturbing evidence that Syrian Kurdish forces were coordinating with the Syrian government and the Russian air force.

Turkey, a major sponsor of the insurgency against Assad, said it welcomed plans for the halt to fighting but was not optimistic about a positive outcome to talks on a political transition.

Turkish Deputy Prime Minister Numan Kurtulmus said Ankara had reservations about actions that Russian forces could take against Syria’s moderate opposition and civilians. Turkey is worried about the expansion of YPG influence in Syria, fearing it could fuel separatism among its own Kurdish population.

A rebel fighter in the Aleppo area said he did not expect the ceasefire plan to work.

“The Russian jets will not stop bombing on the pretext of Nusra and the Islamic State organization, and will keep bombing civilians and the rest of the factions with this pretext,” said Abu al-Baraa al-Hamawi, a fighter with the Ajnad al-Sham group.

“Everything that is happening is pressure to extend the life of the regime,” he told Reuters from the Aleppo area.

(Additional reporting by Kinda Makieh in Damascus, Orhan Coskun in Ankara, Guy Faulconbridge in London and Stephanie Nebehay in Geneva; Writing by Tom Perry; Editing by Giles Elgood)