Hopes of a ceasefire in Syria were fading on Tuesday, three days before an internationally-supported truce as set to be implemented.

Syrian President Bashar al-Assad said the proposal for a so-called "cessation of hostilities" - put forward by the US and Russia in Munich on Friday - would be difficult to implement by Friday.

Even if it was successfully enforced, "it doesn't mean that each party will stop using weapons," Assad said.

"A ceasefire must mean stopping terrorists from strengthening their positions. Moving weapons, equipment, terrorists or strengthening positions must all be forbidden," the Syrian president added.

Rising death toll

The decline in confidence on Tuesday came as the United Nations confirmed that the death toll, following a series of airstrikes on hospitals and schools in Syria on Monday, had risen to 50.

UN Secretary General Ban Ki-Moon said the strikes violated international law and "cast a shadow" over efforts to end Syria's five-year civil war.

French Foreign Minister Jean-Marc Ayrault also condemned the attacks, saying they constituted "war crimes."

"Attacks against facilities in Syria by the regime or its supporters are unacceptable and must stop immediately," Ayrault said.

Observers blame Russia

Officials are yet to confirm who was responsible for Monday's attacks, although the Britain-based Syrian Observatory for Human Rights blamed Russian warplanes for the attack.

Moscow has been carrying airstrikes in support of Syrian President Bashar al-Assad's forces since September 30, 2015 and has helped Syrian troops make considerable territorial gains in recent weeks, particularly in the country's northern province of Aleppo. Their aim is to recapture Syria's largest city, Aleppo, which had a prewar population of more than 2.3 million residents.

Syria's ambassador to Russia claimed on Monday, however, that US planes were responsible for the air strikes on the city.

"Concerning the hospital which was destroyed, in actual fact, it was destroyed by the American Air Force. The Russian Air Force has nothing to do with it," Ambassador Riad Haddad told Rossiya 24 television.

YPG 'threat'

The UN Security Council was also due on Tuesday to discuss Turkey's shelling of Syrian targets. The talks follow a request from Moscow, which has raised concerns over Turkish military attacks on Russian-backed Kurdish militia fighters.

Turkey sees the Kurdish YPG units as a direct threat to its safety and is anxious about the group's recent territorial gains near the Turkish border. The country's officials also claim that the shelling is merely a response to incoming fire from the Kurdish positions.

More than 260,000 people have died since the war began in Syria in 2011. Millions have fled their homes with over 2.7 million seeking refuge in neighboring Turkey. Hundreds of thousands have made their way to Europe, creating a massive refugee crisis.