Barack Obama has phoned Vladimir Putin in a desperate attempt to hold together the Syrian peace deal as Russia launched major airstrikes in the region and Putin was accused of exacerbating the refugee crisis to undermine the European project.

As world leaders scrambled to prevent a Syrian ceasefire from collapsing, the US President contacted the Russian President on Sunday night to urge him to stop bombing “moderate” rebels in the war-torn region in support of its Syrian ally, President Bashar al-Assad.

The White House said: “President Obama emphasised the importance now of Russia playing a constructive role by ceasing its air campaign against moderate opposition forces in Syria.” The Kremlin added that they had a “frank and business-like” discussion with Mr Obama.

Major powers agreed on Friday to a limited cessation of hostilities in Syria, but the deal does not take effect until the end of this week and was not signed by any warring parties - the Damascus government and numerous rebel factions fighting it. The ceasefire would exclude military operations against the so-called Islamic State terror group.

There is little optimism, however, that the deal will do much to end a war that has lasted five years and cost around 250,000 lives.

Russian bombing raids directed at rebel groups are helping the Syrian army to achieve what could be its biggest victory of the war in the battle for Aleppo, the country's largest city and commercial centre before the conflict.

The intensified air campaign follows accusations from Senator John McCain, chairman of the US Senate armed services committee, that Russian President Vladimir Putin was intentionally stoking the refugee crisis in order to undermine the European project.

The International Rescue Committee added that more than 40,000 had fled the countryside of northern Aleppo province in the past two weeks, crowding into towns near the Turkish border.

In pictures: Russian air strikes in Syria Show all 19 1 /19 In pictures: Russian air strikes in Syria In pictures: Russian air strikes in Syria Syrian boys cry following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Aleppo Getty In pictures: Russian air strikes in Syria Russian defense ministry spokesman Major General Igor Konashenkov speaks to the media in Moscow, Russia. Konashenkov strongly warned the United States against striking Syrian government forces and issued a thinly-veiled threat to use Russian air defense assets to protect them AP In pictures: Russian air strikes in Syria Syrians wait to receive treatment at a hospital following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Alepp Getty In pictures: Russian air strikes in Syria Russian Deputy Defense Minister Anatoly Antonov speaks at a briefing in the Defense Ministry in Moscow, Russia. Antonov said the Russian air strikes in Syria have killed about 35,000 militants, including about 2,700 residents of Russia AP In pictures: Russian air strikes in Syria Jameel Mustafa Habboush, receives oxygen from civil defence volunteers, known as the white helmets, as they rescue him from under the rubble of a building following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Aleppo Getty In pictures: Russian air strikes in Syria Civil defence members rest amidst rubble in a site hit by what activists said were airstrikes carried out by the Russian air force in the town of Douma, eastern Ghouta in Damascus, Syria Reuters In pictures: Russian air strikes in Syria A girl carrying a baby inspects damage in a site hit by what activists said were airstrikes carried out by the Russian air force in the town of Douma, eastern Ghouta in Damascus, Syria Reuters In pictures: Russian air strikes in Syria Civilians and civil defence members look for survivors at a site damaged after Russian air strikes on the Syrian rebel-held city of Idlib, Syria Reuters In pictures: Russian air strikes in Syria Civilians and civil defence members carry an injured woman on a stretcher at a site damaged after Russian air strikes on the Syrian rebel-held city of Idlib, Syria Reuters In pictures: Russian air strikes in Syria Volunteers from Syria Civil Defence, also known as the White Helmets, help civilians after Russia carried out its first airstrikes in Syria In pictures: Russian air strikes in Syria The aftermath of Russian airstrike in Talbiseh, Syria In pictures: Russian air strikes in Syria Smoke billows from buildings in Talbiseh, in Homs province, western Syria, after airstrikes by Russian warplanes AP In pictures: Russian air strikes in Syria Russian Air Forces carry out an air strike in the ISIS controlled Al-Raqqah Governorate. Russia's KAB-500s bombs completely destroy the Liwa al-Haqq command unit In pictures: Russian air strikes in Syria Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria Â© TASS/ITAR-TASS Photo/Corbis In pictures: Russian air strikes in Syria Russia claimed it hit eight Isis targets, including a "terrorist HQ and co-ordination centre" that was completely destroyed In pictures: Russian air strikes in Syria A video grab taken from the footage made available on the Russian Defence Ministry's official website, purporting to show an airstrike in Syria In pictures: Russian air strikes in Syria A release from the Russian defence ministry purportedly showing targets in Syria being hit In pictures: Russian air strikes in Syria Russia launched air strikes in war-torn Syria, its first military engagement outside the former Soviet Union since the occupation of Afghanistan in 1979. Russian warplanes carried out strikes in three Syrian provinces along with regime aircraft as Putin seeks to steal US President Barack Obama's thunder by pushing a rival plan to defeat Isis militants in Syria In pictures: Russian air strikes in Syria Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria, a thousand kilometres away. The targets include ammunition factories, ammunition and fuel depots, command centres, and training camps Â© TASS/ITAR-TASS Photo/Corbis

Speaking on the final day of the annual Munich security conference Mr McCain accused Moscow of using its aerial campaign in Syria to add to the flow of people feeling the Middle East.

“He [Mr Putin] wants to exacerbate the refugee crisis and use it as a weapon to divide the transatlantic alliance and undermine the European project,” he said, “His appetite is growing with the eating".

Senator’s McCain’s concerns come amid reports that some Turkish soldiers have entered Syria over the weekend. Turkish Defence Minister Ismet Yilmaz, however, denied the report and said Ankara was not considering sending troops into its neighbour, the state-run Anadolu Agency reported.

Syria's government had said Turkish forces were believed to be among 100 gunmen who entered Syria on Saturday with 12 pick-up trucks mounted with heavy machine guns, in an operation to supply insurgents fighting Damascus.

Senator McCain added that Mr Putin "is not interested in being our partner. He wants to shore up the Assad regime. He wants to re-establish Russia as a major power in the Middle East."

"This is diplomacy in the service of military aggression and it's working because we are letting it.”

On Saturday, Russian Prime Minister Dmitry Medvedev told the Munich conference there was no evidence that Russia was bombing Syrian civilians.