North Sea
© Sputnik/ Vladimir Astapkovich
The submarine, which is believed to be the Kilo-class submarine Stary Oskol, was detected in the sea on Tuesday evening, The Guardian reported.

It is now being escorted by UK Royal Navy frigate HMS Kent and is expected to pass the Strait of Dover later on Wednesday.

"Locating this submarine was a combined effort with NATO allies and shadowing such units is routine activity for the Royal Navy. We continue to escort the submarine as it conducts its passage, providing a visible presence," Cmdr. Daniel Thomas, HMS Kent's commanding officer, said as quoted by the newspaper.

The Russian Defense Ministry said Wednesday it would be "strange" if the UK navy failed to notice Russia's Stary Oskol submarine as it slowly traveled toward the Black Sea Fleet.

"The Russian Defense Ministry is surprised that joint UK-NATO efforts were required to discover a submarine traveling in a surfaced position at slow speed, accompanied by a tugboat. It would have been strange if the Royal Navy together with its NATO allies failed to notice the submarine, particularly considering the fact that our sailors have been greeted โ€” according to an old tradition โ€” by passing merchant vessels from different countries in the Barents, Norwegian and North seas over the past days," the ministry said in a statement.

Last month, the UK Defense Ministry said the Royal Air Force jets had escorted three aircraft claimed to be Russian military transport planes over the Baltic republics but did not disclose the date of the incident.