The United States and Russia have agreed on a "freeze" in fighting along two major fronts in Syria, but not in war-ravaged Aleppo, the Syrian and Russian militaries said Friday.

In a statement carried on state television, Syria's armed forces said the freeze would begin at 1:00 am on Saturday (2200 GMT Friday).

It would last for 24 hours in Damascus and the nearby rebel bastion of Eastern Ghouta, and for 72 hours in the coastal province of Latakia, the heartland of President Bashar al-Assad's Alawite sect.

There was no mention of Aleppo, where a week of fighting has killed more than 200 civilians. "This announcement came after a request from the Americans and the Russians, who met in Geneva to calm down the situation in Damascus and Latakia," a security source in Damascus told AFP. "The Americans asked for Aleppo to be included, but the Russians refused," the source said.

Russia is a key backer of Assad's regime, while the US has supported various opposition factions in the country. A diplomatic source quoted by Russian state news agency RIA Novosti said that Moscow and Washington, co-chairs of the International Syria Support Group, "are the guarantors of the 'regime of silence' implementation by the sides."

The source said the freeze would take effect at midnight Friday. The reason for the discrepancy was unclear.

Russian Lieutenant-General Sergei Kuralenko, based in the Hmeimim airbase in Latakia province, said that during the freeze, "all combat and using any weapons will be forbidden". "We call upon all parties interested in establishing peace on Syrian land to support the Russian-American initiative and not disrupt the 'regime of silence'," he said, quoted by RIA Novosti.

In February, the US and Russia brokered a partial truce in Syria between regime forces and non-militant rebels.

Although violence dropped across large parts of the country, fighting against the Islamic State militant group and Al-Qaeda affiliate Al-Nusra Front continued in Latakia, the eastern province of Deir Ezzor and elsewhere.

Eastern Ghouta is held by the powerful Jaish al-Islam (Army of Islam) rebel group, which has signed on to the truce.

But fighting there between Jaish al-Islam and regime forces has been building in recent weeks. Opposition factions in Eastern Ghouta and Latakia were not immediately available for comment on the freeze. More than 270,000 people have been killed since Syria's conflict erupted in March 2011 with anti-government protests.

President Barack Obama said he expected the United States would meet a goal of admitting 10,000 Syrian refugees before the end of the year despite delays and opposition from critics concerned about security implications.

As Europe grappled with Syrians fleeing the country's civil war last autumn, Obama promised to admit 10,000 Syrian refugees by the end of fiscal year 2016. But the State Department reported on March 31, halfway into the fiscal year, that only 1,285 Syrians had been admitted into the United States. "We're going to keep on pushing," Obama said when asked on Thursday whether the goal would be achieved.

Regime aircraft pounded rebel areas of Syria's second city Aleppo on Friday, hitting a clinic just days after a strike destroyed a hospital, killing two doctors and sparking an international outcry.

More than 200 civilians have been killed in Aleppo over the past week as rebels have pounded government-held neighbourhoods with rocket and artillery fire and the regime has hit rebel areas with air raids.

The bloodshed has brought a February 27 ceasefire between government forces and non-militant rebels to the verge of collapse and raised fears of a humanitarian crisis in the northern metropolis and other battleground areas.A nurse was among several people wounded when the air strike hit the clinic in the rebel-held Al-Marja neighbourhood, the civil defence known as the White Helmets said.

The clinic, which had been providing dental services and treatment for chronic illnesses for about five years, was badly damaged. An AFP photographer said he heard nearly a dozen air raids within the space of a few minutes, followed by the wail of ambulances.

"The planes didn't sleep and didn't let us sleep either," one resident of the densely populated Bustan al-Qasr district told AFP. "The earth is shaking beneath our feet."

At least two civilians were killed in the strikes on Friday, one of them a child, the Syrian Observatory for Human Rights said.

The rebels bombarded government-held areas with rocket and artillery fire, killing three people as they were leaving a mosque after the main weekly prayers, state television reported.

In rebel areas, Friday prayers were cancelled because of the air strikes. It was the second time this week that an air strike had hit one of the few medical facilities still operating in rebel areas.

Late on Wednesday, air strikes hit the Al-Quds hospital and a nearby block of flats in the Sukkari neighbourhood, killing 30 people, including one of the last paediatricians still working in the east of the city.

US Secretary of State John Kerry expressed "outrage" over the hit on the hospital, saying it appeared to be "a deliberate strike on a known medical facility." He called on Moscow to press its Damascus ally "to stop attacking civilians, medical facilities, and first responders, and to abide fully by the cessation of hostilities."

Al-Quds was supported by both Doctors Without Borders and the International Committee of the Red Cross. The ICRC has warned that Aleppo is "on the brink of humanitarian disaster".

"Everyone here fears for their lives and nobody knows what is coming next," said Valter Gros, who heads the ICRC's Aleppo office.

Thursday was the deadliest day in Aleppo since the violence flared last week, with 54 civilians killed, according to the Observatory.

"It is the worst day in Aleppo in five years. The regime did not spare a single neighbourhood," one resident told AFP.