Russian warships off the coast of Norway are carrying fighter bombers that will likely reinforce a final assault on the besieged Syrian city of Aleppo in two weeks, a senior NATO diplomat said on Wednesday citing Western intelligence.

"They are deploying all of the Northern fleet and much of the Baltic fleet in the largest surface deployment since the end of the Cold War," the diplomat said on condition of anonymity.

"This is not a friendly port call. In two weeks, we will see a crescendo of air attacks on Aleppo as part of Russia's strategy to declare victory there," the diplomat said.