A battle group including Russia’s only aircraft carrier, the Admiral Kuznetsov, is reportedly refuelling at sea off North Africa en route to Syria.
Plans for some of the ships to dock at a Spanish port were cancelled after Nato allies voiced concern.
Nato is concerned planes form the carrier could be used to attack civilians in the Syrian city of Aleppo.
The naval battle group has been sailing for the past week from Russia to the Mediterranean.
The Kuznetsov can carry dozens of fighter bombers and helicopters.
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The BBC’s Jonathan Marcus reports from Brussels that the battle group is currently at anchor off the North African coast and taking on fuel.
The group has two “oilers” (tankers) with it and its is not clear which of the vessels is actually refuelling, our world affairs correspondent adds.
The Russian embassy in Madrid formally withdrew a request to Spain for refuelling after being approached by the Spanish foreign ministry.
“Given the information which appeared on the possibility that these ships would participate in supporting military action in the Syrian city of Aleppo, the ministry of foreign affairs requested clarification from the embassy of the Russian Federation in Madrid,” the Spanish foreign ministry had said on Wednesday in a statement to the BBC.
It added that permission had been granted in September for three Russian ships to dock in the port of Ceuta between 28 October and 2 November. It said such stops for Russian naval vessels had taken place for years in Spanish ports.
Nato had said the final decision on resupply rested with Spain.
“We are concerned and I have expressed that very clearly about the potential use of this battle group to increase Russia’s ability and to be a platform for air strikes against Syria,” Nato’s Secretary-General Jens Stoltenberg told journalists on Tuesday.
The naval group also includes a nuclear-powered battle cruiser, two anti-submarine warships and four support vessels, probably escorted by submarines.
The battle group is expected to join about 10 other Russian vessels already off the Syrian coast.
Some 2,700 people have been killed or injured since the Russian-backed Syrian offensive started last month, according to activists.
Western leaders have said Russian and Syrian air strikes on Aleppo could amount to war crimes, an accusation rejected by Russia.
About 250,000 civilians who live in Aleppo have been trapped by the fighting. Moscow announced last week a “humanitarian pause” in attacks as part of a plan to allow civilians and fighters to leave the area.
Russia Syria: Battle group ‘refuels off North Africa’}