A Russian submarine's 'unusual' behavior has NATO's attention. A top official says it's 'limping home.'

"NATO forces have been tracking the Russian Kilo-class submarine, identified as the Novorossiysk, as it travels back to its home port from a patrol in the Mediterranean Sea. The alliance official, who spoke on the condition of anonymity to discuss the developments, said it isn't optimal for a vessel to sail on the surface. A submarine's best defense is its stealth, and these vessels are most vulnerable when surfaced."

"French, Belgian, and other NATO forces tracked the sub last week as it moved north from the Mediterranean off the coast of Western Europe. On Saturday, the Dutch military said it escorted the warship and a supporting Russian tugboat through the North Sea. The Netherlands said escorts prevent Russian vessels from sabotaging critical undersea infrastructure, which has been a point of concern for European countries for years."

"Cdr. Arlo Abrahamson, a spokesperson for NATO's Allied Maritime Command, said the Russian submarine has surfaced "numerous times" during its journey from the Mediterranean back to its home port. "Allies have been keeping a close watch on this submarine during its transit," he told Business Insider in a statement on Monday, and the UK said Tuesday that tracking the Novorossiysk has been a coordinated operation involving roughly a dozen warsh"

The Russian Kilo-class submarine Novorossiysk traveled largely on the surface while returning from a Mediterranean patrol, prompting NATO monitoring. Allies observed numerous surfaceings and described the vessel as appearing broken and 'limping home.' French, Belgian, Dutch and other NATO units tracked the submarine northward, and the Dutch military escorted the sub and a supporting Russian tug through the North Sea. NATO officials said surface transit is far from ideal because submarines rely on stealth and are most vulnerable when surfaced. The Netherlands said escorts aim to protect undersea infrastructure. Tracking involved coordinated operations by roughly a dozen warships.

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