BEIRUT (Reuters) - A nationwide ceasefire in Syria brokered by the United States and Russia went into effect 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 p.m. (11.00 a.m. ET), 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 issued a joint statement listing deep reservations with the agreement they described as unjust, echoing concerns outlined in a letter to the United States on Sunday. While the statement did not explicitly back the ceasefire, rebel sources said the groups were abiding by it.

“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,” Zakaria Malahifji of an Aleppo-based rebel faction told Reuters.

Combatant sources on both sides said calm prevailed in the first hours of the ceasefire but reported violations increased later in the night.

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

U.S. Secretary of State John Kerry said early reports suggested there had been some reduction in violence.

He told reporters at the State Department it was too early to draw a definitive conclusion about how effective the truce will be and that there would no doubt be some reports of violations “here and there”.

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.

Besher Hawi, a resident of the opposition-held city, said it had been calm since the ceasefire came into force, after a heavy day of bombardment.

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“It’s excellent but I certainly have no confidence in the regime. It could bomb at any moment,” he told Reuters from Aleppo, speaking via a web-based messaging system.

Residents of government-held western Aleppo, frequently hit by rebel shelling, also expressed doubt over whether the truce would last. “Every time there’s a truce the militants ... hit us,” said an Aleppo resident who gave his name as Khaled. “We hope things will improve. May the army be victorious.”

As the night went on, the Syrian Observatory for Human Rights, a Britain-based monitoring group, reported government air strikes and bombardment in Hama and Aleppo provinces, and shelling of rebel-held areas near Damascus, though it said the violations were not serious. A rebel in the Aleppo area said warplanes had opened fire with machine guns north of the city.

A Syrian military source meanwhile said armed groups in Aleppo had sniped on residential buildings, and fired three mortar bombs at a government-held area on the city outskirts.

The ceasefire 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.

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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-hardline jihadist group that controls swathes of Syria and neighboring Iraq.

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

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 kilometers from Damascus, has helped the government secure important areas to the southwest 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 coordination 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.