A nationwide ceasefire started at sunset Monday in Syria as part of an ambitious plan sealed by the United States and Russia to reduce violence in the war-torn country and relaunch a peace process.

More than an hour after the truce started at 7 pm (1600 GMT), calm prevailed in most parts of the country, the Syrian Observatory for Human Rights, a monitoring group, reported.

However, government forces continued shelling in rebel-held areas of the north-western province of Idlib, the watchdog said.

It reported an exchange of shelling by the regime and opposition forces in Quneitra in southern Syria.

The Syrian Army said it would observe a seven-day ceasefire, according to Syria's state news agency SANA.

Government troops said they still "reserve the right for a firm response to any violation from armed groups," the agency reported, citing a statement by the army command.

Warplanes ceased flying over Aleppo, Abdel Mounein Juneid, an activist based in the divided northern city, told dpa.

"I can say that since 7 pm we haven't heard any plane or any artillery shelling," said Ibrahim al-Hajj, a member of the White Helmets, a volunteer civil defence organization that works mainly in opposition areas.

In Washington, US Secretary of State John Kerry said initial signs of the ceasefire were promising.

"The earliest reports are that there is some reduction in violence," he said at the State Department.

The cessation of hostilities excludes only al-Qaeda terrorist network-linked group al-Nusra and the Islamic State movement.

It was still "far too early to draw any definitive conclusions, and I'm not drawing any conclusions," Kerry said. "There will undoubtedly be reports of a violation here or there."

Russia, a major ally of Syrian President Bashar al-Assad, said it would continue to conduct airstrikes against "terrorists in Syria," referring to the Islamic State radical group and al-Qaeda-affiliated militants.

Russia and the US would coordinate the airstrikes via a joint centre of operations that is currently being set up, Russian General Sergei Rudskoi said, according to comments carried by the Interfax news agency.

The opposition Syrian National coalition called the truce "a step in the right direction," saying that its affiliated Free Syrian Army will "positively deal with it."

However, the moderate opposition grouping demanded protection for Syrian civilians from attacks by the regime and its allies.

The US-backed coalition called for a clear framework for monitoring the truce.

Syria's main Kurdish group, the People's Protection Units, welcomed the ceasefire.

"We are optimistic with this deal. We will be able to cooperate in the fight against the terror of Daesh," the US-backed group added in a statement, using an Arabic acronym for Islamic State. 

Hours before the start of the truce, at least 13 civilians were killed in airstrikes by unidentified warplanes attacking rebel-held areas in the north-western Idlib province, the Observatory said.

The regime and opposition forces also were exchanging fire in Aleppo, a major battleground in Syria's five-year conflict.

Shelling by government forces on rebel-held areas in the Eastern Ghouta region on the outskirts of Damascus left at least one civilian dead and 10 injured, the Observatory added.

Earlier on Monday, al-Assad vowed that his troops would regain territory from rebels "without hesitation or relenting," fuelling misgivings about the truce.

Iranian President Hassan Rowhani meanwhile said Iran would do everything it can to make peace possible in Syria.

The US and Russia sealed an agreement Friday on Syria that also envisages unprecedented military coordination against militant groups there. The truce is the second this year brokered by Washington and Moscow.

Syria's conflict started with peaceful anti-government protests in March 2011. It spiralled into a full-blown war that has claimed more than 250,000 lives, according to activists' estimates.

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