NATO chief Jens Stoltenberg expressed concern Thursday that a Russian aircraft carrier battlegroup could join attacks on Syria's besieged city of Aleppo.

Warships from NATO member Britain were shadowing the battle group through the North Sea and Stoltenberg said NATO navies would closely monitor the warships in "a responsible and measured" way as it headed to the Mediterranean.

"We are concerned Russia's carrier group will support military operations in Syria in ways which increase human and civilian suffering," Stoltenberg said at NATO headquarters in Brussels.

"What creates concern now is that this group may be used to... increase attacks on Aleppo," the former Norwegian premier said after talks with Ukrainian President Petro Poroshenko.

"NATO navies will in a normal way monitor Russian ships and do so in a responsible and measured way as they always do because this is the way we follow Russian military actions," he added.

A Royal Navy spokesman told AFP earlier the eight-strong group including the Admiral Kuznetsov aircraft carrier was the biggest deployment of Russian naval assets seen off British shores in recent years.

Britain's HMS Richmond frigate and the HMS Duncan air defence destroyer were monitoring the task group, which included the nuclear-powered Pyotr Veliky battle cruiser and the Vice-Admiral Kulakov destroyer.

The deployment comes after Defence Minister Sergei Shoigu said the Admiral Kuznetsov, part of its Northern Fleet, would be sent to the eastern Mediterranean to boost Russian naval forces in the area.

Russia launched a bombing campaign in September last year in support of long-time Syrian President Bashar al-Assad and deployed a naval contingent to back up the operations which have focused on rebel positions in Aleppo.