A ceasefire began at sunset aimed at ending the five-year long civil war in Syria that has killed over a quarter of a million Syrians and forced five million more to flee the country. Under an agreement between the US and Russia, the initial truce will last seven days, during which time UN convoys will bring aid to besieged districts and Syrian government bombing of civilian areas will stop.

The Syrian army said a seven-day “calm” would be applied across Syria from 7pm on Monday, but it reserved the right to respond decisively “using all forms of fire to any violation by the armed groups”.

If the truce holds for a week, the US and Russia will form an unprecedented military partnership that will control airstrikes targeting Isis and Jabhat al-Nusra, the former Syrian affiliate of al-Qaeda that has relabelled itself as Jabhat Fateh al-Sham. Moderate rebel groups are supposed to separate themselves from al-Nusra in order to avoid air attack, though there are doubts about the feasibility of this.

Syria's President Assad celebrates Eid al-Adha at mosque in Daraya, site of one of his most brutal sieges

Most of the participants in the war say they will respect the ceasefire, though armed groups seeking to overthrow President Bashar al-Assad said they did not think al-Nusra should be singled out as a target for air strikes.

Observers in Damascus say one of the weaknesses of the accord is that “Nusra is the backbone of the armed opposition, but has no reason to abide by the terms of the ceasefire from which it is excluded”.

In the hours before the ceasefire, Syrian air force planes continued to bomb – having killed 90 civilians in Idlib province and Aleppo on Saturday – and al-Qaeda-linked groups conducted an offensive in the south of Syria. Opposition critics of the deal say that it lacks credible monitoring mechanisms.

Mr Assad has agreed to the accord, but said on Monday that his government would take back all of Syria from “the terrorists”. Earlier, he had prayed as part of the Eid al-Adha holiday in the main mosque in Daraya, a pro-rebel district of Damascus that had withstood a Syrian Army siege for four years until it surrendered on terms last month. Its pre-war population of 200,000 has disappeared and its rubble-strewn streets are silent and empty.

Mr Assad had little choice but to agree to a deal that was negotiated by Russia, on whom he depends for air support, weapons and diplomatic backing.

But, in any case, the agreement has several advantages for him, such as freezing the military situation as his military forces tighten their grip on Damascus and rebel-held East Aleppo. The US and Russia will be targeting al-Nusra, which has led all successful offensives by the armed opposition in recent years. The Syrian Army will be freed up to attack Isis, which still holds swathes of territory in eastern Syria.

The Syrian Kurds announced they were adhering to the truce yesterday, though not, presumably, when it comes to fighting Isis, which is excluded from the ceasefire. The People’s Protection Units (YPG) have provided the infantry which has won a series of battles against Isis in the last 18 months because of their military skill and discipline and because they can call in the devastating firepower of the US air force to destroy Isis targets.

A feature of the war in Syria and Iraq is that the anti-Isis and anti-al-Nusra armies – the YPG, Syrian Army, Iraqi Army and Iraqi Kurdish Peshmerga – all rely on foreign air forces.

This makes it difficult for them to go against whatever their foreign allies want them to do politically. Isis and al-Nusra are by far the most militarily effective parts of the ant-Assad opposition, primarily because of their mass use of suicide bombers which makes it possible for them to fight better-equipped enemies.

Rebel groups that have clashed with al-Nusra in the past have been rapidly defeated and there is no reason to believe that they will be more successful in similar circumstances today.

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UN aid convoys will enter besieged villages, towns and urban enclaves as an important part of the agreement, bringing relief to between 250,000 and 275,000 people in East Aleppo.

These have been cut off over the last week after a government counter-offensive retook the Ramouseh Road in the south of the city. Under the agreement, neither government nor rebels are allowed to impede traffic on this road, though it remains in government hands.

The famous Castello Road in north Aleppo, previously the main rebel supply line to East Aleppo, will be demilitarised with both side’s forces pulling back.

There are some 592,000 Syrians besieged or blockaded, most of them in opposition areas, though 110,000 are trapped by Isis in the eastern provincial capital of Deir Ezzor. Conditions differ markedly from place to place.

A woman from the rebel-held town of Madaya west of Damascus said she and her children were reduced to eating boiled thistles and putting spices in hot water to give it taste. But the 282,000 people in Eastern Ghouta, much the largest rebel enclave in greater Damascus, are better fed because they have agricultural land, and smugglers can bring in goods by tunnels or by bribing government checkpoints.

The chances of the truce turning into a long-term ceasefire are greater than before because the US and Russia are the most powerful countries engaged in the Syrian war.

The US has been using its air force in the country since 2014 and the Russians a year later. They are in a position to influence local allies on the ground in Syria and regional powers like Turkey, Saudi Arabia, Iran and Qatar. While the compliance of these countries, cannot be guaranteed even when they have publicly endorsed the agreement, they will have to be circumspect in trying to derail it.

Saudi Arabia and Qatar face the failure of their long campaign to overthrow Mr Assad and the targeting of al-Nusra, to whom they have given weapons and money.

Turkey has done so too, but lack of international criticism of its incursion into northern Syria in August may be a quid pro quo for Ankara not seeking to undermine the latest accord. Iran and Hezbollah of Lebanon have approved the deal and will be pleased that Mr Assad and his government are more secure, but they may also fear the outcome of US-Russian conciliation which might reduce their influence.