The Alaed cargo vessel will return to Murmansk on Saturday and then set off again for the Syrian port of Tartus under the accompaniment of at least one other Russian ship, an unnamed diplomatic source told the news agency.

"If it sails on its own, something unpleasant could happen that they blame on pirates or a storm," the source said in reference to a possible foreign military attempt to intercept the cargo vessel.

The source stressed that the ship would not be accompanied by navy vessels "in order to avoid the unnecessary involvement of the Russian navy in this complicated and ambiguous situation."

Russia says the Mi-25 helicopters were being returned to Syria after undergoing repairs at Russian factories under contracts that could not be breached.

The ship's cargo has greatly troubled the Syrian regime's Western and Arab foes amid calls on Moscow to halt all military cooperation with Damascus due to the government's crackdown on the opposition.

The Alaed was forced to return from its journey after its mission was reported by the US State Department, prompting the British insurer to withdraw its coverage.