Three Portsmouth-based warships have been on alert this week, tracking seven warships and a submarine passing through the English Channel.
HMS Tyne, Severn and Mersey were called up to shadow seven vessels from the Russian Federations Navy, as well as an Algerian submarine sailing past the Kent coast.
Senior officers said the River-class patrol ships reacted "at short notice" ensuring "the safe transit" of the visiting vessels.
A "pleasant duty to welcome our Algerian friends"
The task of escorting the Algerian submarine fell to the crew of HMS Severn.
The ships is normally used for training Royal Navy navigators, but is also part of the offshore patrol fleet used to protect home waters.
The Algerian boat was passing through the Channel on its way back to Africa.
Severn's commanding officer, Commander Philip Harper, said: "It has been a pleasant duty to welcome our Algerian friends for their transit of UK waters in great weather as they head home."
Russian ships operated "in a safe and professional manner"
HMS Mersey and her crew was tasked with escorting three vessels – frigate Admiral Kasatonov, a supporting tug Nikolay Chiker and tanker Vyazma – through the Channel and the Dover Strait into the North Sea.
High winds and large sea states disrupted the journey, as the Russian ships chose to shelter in calmer waters before resuming their journey.
Mersey's navigator, Lieutenant Thomas Bees, said that “the Russian Federation naval vessels operated in a safe and professional manner throughout their transit.”
Before the Kasatonov group sailed through the Channel Mersey worked in tandem with HMS Tyne to keep constant watch on four Russian vessels sailing through the Channel towards the Atlantic.
The quartet – three Ropucha-class amphibious ships capable of landing tanks, Minsk, Kaliningrad and Korolev, and the frigate Boiky – were located in the North Sea and closely followed through the Dover Strait and into the English Channel before reaching the open waters of the North Atlantic.
As part of the operation, the Portsmouth-based offshore patrol ships worked with several allied NATO ships and aircraft to keep track of the movements of the Russian group.
What do the River-class patrol vessels do?
The Royal Navy says that Portsmouth-based Mersey and her sister Tyne are tasked with making sure fishing vessels operating in home waters abide by the rules and stick to UK regulations in the country’s Exclusive Economic Zone on behalf of the Marine Management Organisation.
The third ship, HMS Severn, is focused mainly on training the next generation of navigators for the Royal Navy.
The Navy says that the River class typically spends around ten months of every year at sea, acting as the eyes and ears of the government in UK waters.