The US Navy's top admiral has warned China that the US could treat China's coast guard and maritime militia the same as the People's Liberation Army Navy, the Financial Times reported.

"The US Navy will not be coerced," Chief of Naval Operations Adm. John Richardson told his Chinese counterpart, Vice Adm. Shen Jinlong, in January.

The move is a blow to Chinese gray-zone aggression, a tactic China uses to exercise its will in the South China Sea and the East China Sea without escalating to armed conflict.

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The US Navy has reportedly warned China that it could treat the Chinese coast guard and the paramilitary fishing fleet known as the maritime militia the same as the Chinese navy — as combatants.

Adm. John Richardson, the chief of naval operations, "made it very clear" to his Chinese counterpart in January that "that the US Navy will not be coerced," the Financial Times reported, citing an interview with the Navy's top admiral.

"By injecting greater uncertainty about how the US will respond to China's grey-zone coercion, the US hopes to deter Chinese destabilizing maritime behavior," Bonnie Glaser, a China expert at the Center for Strategic and International Studies (CSIS), told the Financial Times.

The warning is a blow to Chinese "gray-zone aggression," which involves using non-naval vessels to exercise Beijing's will in disputed waterways, such as the South China Sea and the East China Sea, without risking escalation.

A Chinese coast-guard vessel sails near disputed East China Sea islands. 11th Regional Coast Guard Headquarters of Japan/AP

The Chinese coast guard and the maritime militia are China's frontline forces in contested waters. They have been involved in intimidating and, at times, even attacking other ships.

Between 2010 and 2016, the Chinese coast guard was involved in 71% of 45 incidents in the South China Sea, according to CSIS. The coast guard was placed under the control of China's Central Military Commission last year.

China's coast-guard ships are much more heavily armed than traditional coast-guard vessels — some experts have described them as navy "gray hulls" that have been painted white.

Often operating alongside the coast guard is the shadowy maritime militia, which has frequently been involved in harassing foreign vessels.

For years, the maritime militia operated under the cloak of plausible deniability.

"Make no mistake," Andrew Erickson, an expert on the Chinese maritime militia at the Naval War College, told lawmakers in 2016, "These are state-organized, -developed, and -controlled forces operating under a direct military chain of command."

Dozens of Chinese fishing vessels surrounded a Vietnamese ship. One then rammed it, causing it to sink, as can be seen in this 2014 photo. REUTERS/Stringer

Erickson has repeatedly urged the US military to call the maritime militia what it is — a paramilitary force, one trained and directed by the Chinese military.

The Department of Defense for the first time dragged this paramilitary force out of the shadows in 2017 in its annual report on Chinese military power. The Pentagon said the maritime militia is used to "enforce maritime claims and advance [China's] interests in ways that are calculated to fall below the threshold of provoking conflict."

In the 2018 report, the department said that the maritime militia "plays a major role in coercive activities to achieve China's political goals without fighting."

Now it appears the US military is taking things a step further. The Department of Defense has not only recognized the existence of these forces, but it is also threatening to treat these forces as combatants should they engage in such behavior.

"It is a warning shot across the bow," James Stavridis, a retired US admiral, told the Financial Times. "A combatant is a combatant is the message."

This bold move is a step toward curbing Chinese gray-zone aggression, but it undoubtedly increases the risk of armed conflict in disputed waterways.