BRUSSELS, Oct 26 (Reuters) - NATO Secretary-General Jens Stoltenberg said it was up to Spain to decide whether to refuel a Russian tanker travelling with a carrier battle group heading to Syria but that NATO had expressed its concern to allies including Madrid.

"It is each up to each nation to decide, as has been NATO policy for many years, but we are concerned about the potential use of this carrier group to increase attacks against civilians in Aleppo," Stoltenberg said. "All allies are aware of our concerns."

(Reporting by Robin Emmott) ((philip.blenkinsop@thomsonreuters.com; +32 2 287 6838; Reuters Messaging: philip.blenkinsop.thomsonreuters.com@reuters.net))