US President Barack Obama has told his Russian counterpart Vladimir Putin to stop air strikes against moderate Syrian rebels, says the White House.

In Saturday's telephone call, Mr Obama stressed to Mr Putin the need to relieve the blockade of besieged parts of the war-torn country so humanitarian aid can be allowed in, according to the White House.

He also emphasised the need to implement a nationwide cessation of hostilities in Syria to abide by a truce agreement struck last week, said the statement.

The Kremlin confirmed Mr Putin had a telephone conversation with Mr Obama about the Syrian war, stating the call took place at Washington's initiative.

The Russian president stressed to his US counterpart the importance of creating a "common front against terrorism", said the Kremlin.

Mr Putin also urged a working relationship between the Pentagon and Russian Defence Ministry, according to Moscow.

Major world powers agreed in Munich on Friday to a cessation of hostilities in Syria set to begin in a week and to provide rapid humanitarian access to besieged Syrian towns.

But the Kremlin said it would not stop air attacks in Syria, because the cessation of hostilities doesn't apply to Islamic State or al Qaeda-affiliate al Nusrah.

Aided by Russian air power, Syrian government troops have nearly encircled Aleppo, Syria's second city.

But Western powers say most of Moscow's air campaign has targeted more moderate rebel groups, rather than the jihadists.

Republican presidential candidates and Democratic White House hopeful Hillary Clinton have called for the US to declare a no-fly zone over Syria.

But Mr Obama has been sceptical of the idea, saying that critics of his Syria policy often talk "mumbo jumbo".

The US military has not backed the idea of a no-fly zone, which would require American bombing raids on Syrian air defences.

To enforce the exclusion zone, a substantial number of US combat aircraft would then have to continuously patrol skies currently flown by Russian warplanes.

Syria's civil war has been raging since 2011, but the conflict entered a new phase last year when Russia started providing air support to government forces.

Speaking in the capital Damascus on Thursday, Syrian President Bashar al Assad vowed to retake the entire country.