NATO navies are increasingly worried about Russian submarines.

That concern has led to renewed focus on anti-submarine warfare.

But some of those navies have neglected their ASW capabilities, meaning they have to play catch-up over the past several years.

US and European officials have warned repeatedly in recent years that more sophisticated and more active Russian submarines pose a growing threat, and NATO countries are taking steps to counter that perceived challenge.

Adm. James Foggo, head of US Navy forces in Europe and Africa, has said that a "fourth battle of the Atlantic" — which comes after the naval warfare of World War I, World War II, and the Cold War — is already being fought, and it ranges far beyond the waters of the Atlantic.

"I've used the term in some of my writings that we are in a 'fourth battle of the Atlantic' right now, and that's not just the Atlantic," Foggo said on the first edition of his podcast, "On the Horizon," published at the end of August.

Adm. James Foggo, head of US Naval Forces Europe-Africa, meets officers from the Arleigh Burke-class guided-missile destroyer USS Donald Cook in Spain, January 12, 2018. US Navy/Mass Comm. Specialist 3rd Class M. Jang

"That's all those bodies of water I talked about, the Arctic, the Baltic, the Mediterranean Sea, the Black Sea, and the approaches to the Straits of Gibraltar and the GIUK gap, and the North Atlantic," he added, referring to waters between Greenland, Iceland, and the UK that were a focal point for submarine activity during the Cold War.

While some intelligence estimates from the Cold war indicate that current Russian sub activity is still well below peaks reached during that time, US and European officials have been expressing concern for the past several years.

"The activity in submarine warfare has increased significantly since the first time I came back to Europe and since the Cold War," said Foggo, who previously commanded the Navy's 6th Fleet. "The Russian Federation navy has continued to pump rubles into the undersea domain, and they have a very effective submarine force."

The GIUK Gap has long been seen as a key chokepoint between the North Atlantic and the Arctic Oceans. Google Maps

That force's readiness has also improved to the point where the Russian navy can keep some of them deployed most of the time.

US Chief of Naval Operations Adm. John Richardson told lawmakers earlier this year that Moscow has "really stepped on the gas," with its subs, "both in technology and in ... the amount of time that they're spending abroad."

Russia's newest class of submarines, Yasen-class subs, have drawn comparisons to the US Navy's best subs, and Moscow matches that technical progress with the geographic advantage of being able to deploy from bases on the Barents, Baltic, and Black seas.

Some of Russia's Kilo-class subs, which are newer, more advanced diesel-electric boats, are able to launch Kalibr cruise missiles from those areas and reach "any of the capitals of Europe," Foggo said.

But, he added, the best way to track these boats is not just with other submarines.

The Russian Yasen-class nuclear-attack sub Severodvinsk. Wikimedia Commons

While Foggo was a planner at the Pentagon, Adm. Jonathan Greenert, then the Navy's chief of operations, "would often say, 'Hey, look, the best way to find another submarine is not necessarily with another submarine. That's like a needle in a haystack,'" Foggo said.

A more effective approach draws on the submarine, surface, and air assets to put a full-court press on rival subs.

Anti-submarine warfare "is a combined-arms operation, and let no one forget that," Foggo added, saying that it involved all the US Navy Europe and Africa's assets as well as those of the 6th Fleet, which is responsible for the eastern half of the Atlantic from the Arctic to the Horn of Africa.

NATO navies, and many other navies around the world, have increased their attention to anti-submarine-warfare capabilities in recent years, adding improved technology and spending more time practicing. One sign of that focus has been the growing market for sonobuoys, which are used to hunt targets underwater.

Naval Aircrewman (Operator) 2nd Class Karl Shinn loads a sonobuoy on a P-8A Poseidon, April 10, 2014. US Navy/Chief Mass Comm. Specialist Keith DeVinney

In early 2017, US Navy ships deployed in the eastern Mediterranean engaged in the tricky game of tracking the Krasnodar, a Russian attack sub whose noise-reducing capability earned it the nickname "The Black Hole."

Sailors in the USS George H.W. Bush carrier strike group were tasked with following the elusive Krasnodar, despite having little formal training in anti-submarine operations.

"It is an indication of the changing dynamic in the world that a skill set, maybe we didn't spend a lot of time on in the last 15 years, is coming back," Capt. Jim McCall, commander of the air wing on the USS Bush, told The Wall Street Journal at the time.

Cmdr. Edward Fossati, commander of the Bush strike group's sub-hunting helicopters, told The Journal that improved tracking abilities had helped keep things even with Russian subs' improved ability to avoid detection.

US sailors aboard the USS Vicksburg take part in NATO's Dynamic Mongoose anti-submarine exercise in the North Sea off the coast of Norway, May 4, 2015. REUTERS/Marit Hommedal/NTB Scanpix

But the Navy has had to keep pace in what Navy Secretary Richard V. Spencer has called "a constant foot race."

Navy surface forces let their focus on ASW "wane considerably" in the years after the Cold War, Bryan Clark, a senior fellow at the Center for Strategic and Budgetary Assessments, said in an interview earlier this year.

"Up until a few years ago, their ASW systems were not modernized to deal with new Russian and Chinese subs," said Clark, a former submariner, but the Navy has added new, improved gear, like processors and towed arrays, that have increased their capabilities.

"Surface ships are able to get back into the ASW business," Clark said.