The MV Alaed, a Russian-operated cargo vessel, is currently thought to be sailing through the North Sea after allegedly picking up a consignment of munitions and MI25 helicopters - known as "flying tanks" - from the Russian Baltic port of Kaliningrad.

Washington, which last week condemned Moscow for continuing to arm the Syrian regime, has asked British officials to help stop the Alaed delivering its alleged cargo by using sanctions legislation to force its London-based insurer to withdraw its cover.