A nation-wide ceasefire brokered by the United States and Russia came into effect in Syria on Monday evening, the second attempt this year by Washington and Moscow to halt the five-year-old civil war. The Syrian army, announced the truce at 7 pm (1600 GMT), the moment it took effect, saying the seven-day "regime of calm" would be applied across Syria. It reserved the right to respond with all forms of firepower to any violation by "armed groups".

Rebel groups fighting to topple President Bashar al-Assad had decided to respect the ceasefire, while expressing their deep reservations about the overall agreement, said Zakaria Malahifji of the Aleppo-based rebel faction Fastaqim.

"Regarding a truce, a ceasefire, the delivery of aid, this is a moral question and there is no debate around this, we absolutely welcome this, but there are other articles around which there are reservations," he told Reuters.

Combatant sources on both sides said calm was prevailing in the first hours of the ceasefire. The Syrian Observatory for Human Rights, a Britain-based monitoring group, shared that assessment.

Russia is a major backer of Assad, while the United States supports some of the rebel groups fighting to topple him.

The agreement's initial aims include allowing humanitarian access and joint U.S.-Russian targeting of jihadist groups, which are not covered by the agreement.

Russia's foreign ministry said in a statement that aid to the besieged city of Aleppo would start immediately.

The agreement comes at a time when Assad's position on the battlefield is stronger than it has been since the earliest months of the war, thanks to Russian and Iranian military support. Hundreds of thousands of Syrians have been killed in the conflict and 11 million made homeless in the world's worst refugee crisis.

ASSAD EMBOLDENED

Hours before the truce took effect, an emboldened Assad vowed to take back all of Syria. In a gesture loaded with symbolism, state television showed him visiting Daraya, a Damascus suburb long held by rebels but recaptured last month after fighters surrendered in the face of a crushing siege.

"The Syrian state is determined to recover every area from the terrorists," Assad said in an interview broadcast by state media. Earlier he performed Muslim holiday prayers alongside other officials in a bare hall in a Daraya mosque.

He made no mention of the ceasefire agreement, but said the army would continue its work "without hesitation, regardless of any internal or external circumstances".

The ceasefire is the boldest expression yet of hope by the administration of U.S. President Barack Obama that it can work with Russian President Vladimir Putin to end the war. All previous diplomatic initiatives have collapsed in failure.

The Obama administration opposes Assad but wants to shift the focus of fighting from the multi-sided civil war between Assad and his many foes to a campaign against Islamic State, an ultra-hard-line jihadist group that controls swathes of Syria and neighbouring Iraq.

The ceasefire deal is backed by foreign countries ranging from Assad's ally Iran to Turkey, one of the main supporters of groups fighting to overthrow him.

But maintaining the ceasefire means overcoming big challenges, including separating nationalist rebels who would be protected under it from jihadist fighters who are excluded.

The rebels say the deal benefits Assad, whose military position has improved since the last truce brokered by Washington and Moscow collapsed earlier this year.

The capture of Daraya, a few kilometres from Damascus, has helped the government secure important areas to the south-west of the capital near an air base. The army has also completely encircled the rebel-held half of Aleppo, Syria's largest city before the war, which has been divided into government and opposition-held zones for years.

RUSSIA CONCERNED

In the hours before the ceasefire took effect, fighting raged on several key frontlines, including Aleppo and the southern province of Quneitra. The Observatory said an air strike in rebel-held Idlib province killed at least 13 people.

Under the agreement, Russian-backed government forces and opposition groups are expected to halt fighting for a while as a confidence building measure. Opposition fighters are expected to separate from jihadist groups in areas such as Aleppo.

But distinguishing protected rebels from jihadists is difficult, particularly with regards to a group formerly called the Nusra Front, which was al Qaeda's Syria branch until it changed its name in July.

The group, which now calls itself Jabhat Fateh al-Sham, has been playing a vital role in the battle for Aleppo allied with other rebel factions. It remains excluded from the ceasefire, and other rebel groups say government forces or their allies can use its presence as an excuse to hit other targets.

Russia's foreign ministry said it was concerned that some opposition groups including the powerful Ahrar al-Sham, which fights in close co-ordination with Jabhat Fatah al-Sham, had refused to respect the ceasefire. However, a source in the opposition told Reuters that Ahrar al-Sham, would back the cessation of hostilities in an announcement later on Monday.

Washington has said the ceasefire includes agreement that the government will not fly combat missions in an agreed area on the pretext of hunting fighters from the former Nusra Front. However, the opposition says a loophole would allow the government to continue air strikes for up to nine days.

Nationalist rebel groups, including factions backed by Assad's foreign enemies, wrote to Washington on Sunday to express deep concerns.

The letter, seen by Reuters, said the ceasefire shared the flaw that doomed the previous truce: a lack of guarantees or monitoring mechanisms. It also said Jabhat Fateh al-Sham should be included, as the group had not carried out attacks outside Syria despite its previous ties to al Qaeda. Jabhat Fatah al-Sham said the deal aimed to weaken the "effective" anti-Assad forces, and to "bury" the revolution.