In what defence sources are calling a 'highly unusual surge' of activity, NATO allies have deployed at least 27 specialist submarine-hunting sorties in a bid to track the vessel.

In what defence sources are calling a 'highly unusual surge' of activity, NATO allies have deployed at least 27 specialist submarine-hunting sorties in a bid to track the vessel.

When an intruder was detected near a Nato exercise at the weekend, it sparked a cat-and-mouse game spanning thousands of square miles