A US Navy warning to China that it will treat Beijing's maritime militia the same way it treats Chinese naval vessels has upped the ante in the South China Sea and raised questions on how the new policy will work out in practice.
Beijing's use of non-military vessels has been a worry for claimants to islands in the South China Sea.
China is known to deploy vessels that look like fishing trawlers but are actually part of its maritime militia, operating in a so-called "grey zone" and likely reporting to Chinese navy commanders. This fleet has been strengthened since around 2015, analysts say.
A US Department of Defence report last year noted that the People's Liberation Army (PLA) Navy, the China Coast Guard and the maritime militia "sometimes conduct coordinated patrols", and together form the "largest maritime force in the Indo-Pacific".
The issue is how official naval forces, including and most importantly those of the United States, should react if China uses ostensibly civilian vessels to push its claims in the South China Sea. Already, such vessels have engaged in confrontations, blocking or ramming other countries' boats.
The US has a mutual defence treaty with the Philippines, which has been feeling the pressure from China's maritime militia.
Since early March, Chinese fishing vessels have been operating near two Philippine-held features in the disputed Spratly Islands, as tensions ran high over the more than 200 Chinese ships near Thitu Island, which the Philippines calls Pag-asa. Last month, American and Philippine forces conducted, apparently for the first time, a joint exercise in seizing an island back from an occupying force, the Philippines' GMA News reported.
"If the US is going to support its ally and help deter this kind of predatory grey-zone operation, then it must signal that any maritime force used for what amounts to gunboat diplomacy will be treated like the naval forces they are pretending to be," Dr Patrick Cronin, Asia-Pacific security chair at the Hudson Institute, told The Straits Times.
The issue was apparently raised at a January meeting between US chief of naval operations John Richardson and his Chinese counterpart Shen Jinlong, commander of the PLA Navy.
The Financial Times over the weekend quoted Admiral Richardson as saying in an interview that he had told Vice-Admiral Shen China's maritime militia and coast guard would be treated as China's navy.
"I made it very clear that the US Navy will not be coerced and will continue to conduct routine and lawful operations around the world, in order to protect the rights, freedoms and lawful uses of sea and airspace guaranteed to all," Adm Richardson told the newspaper.
The implications of the change in US policy depends on how it is implemented, Dr Bonnie Glaser, senior adviser for Asia and the director of the China Power Project at the Centre for Security and International Studies, told ST.
"If a Coast Guard or maritime militia ship interferes with a Fonop (freedom of navigation operation), will it be rammed by a naval vessel? Will the US intervene using a navy ship to stop a Chinese Coast Guard or militia boat from harassing a boat from a smaller country?
"The belief has been that the Chinese are confident that the US won't use a grey hull (official navy ship) unless China has, so the US can be kept at bay," Dr Glaser said. "The goal of changing the policy is to deter destabilising behaviour. But, at the end of the day, the US doesn't seek escalation in the South China Sea, so I doubt that it would deploy navy ships against white-hulled vessels in most scenarios."