MANILA - President Rodrigo Duterte on Tuesday (Aug 20) said the Philippines would insist – “in an unfriendly manner”, if need be – that ships of other nations seek clearance first before passing through the country’s waters.

The move comes amid a simmering row between Manila and Beijing over the unannounced passage of Chinese warships through the Philippines' 12-mile territorial sea.

"To avoid misunderstanding in the future, the Philippines is putting on notice that, beginning today, all foreign vessels passing our territorial waters must notify and get clearance from the proper government authority well in advance of their actual passage," said Mr Duterte's spokesman Salvador Panelo.

"Either we get a compliance in a friendly manner, or we enforce it in an unfriendly manner."

Mr Panelo said the instruction came directly from Mr Duterte.

Asked if this meant "military action", Mr Panelo told reporters: "Well, if it will have to take that, we will do it".

The statement is uncharacteristically assertive for Mr Duterte, who has made it a core foreign policy to cultivate warm ties with Beijing.

He had said many times that he did not want to provoke China into an argument or a conflict, as the Philippines would likely pay a heavy price.

Mr Duterte will embark on his fifth visit to China next week.

The visit is expected to be less cordial than previous ones, as he has indicated his intention to discuss with his counterpart Xi Jinping a historic 2016 ruling by an international tribunal on a case lodged by the Philippines that invalidated China's claim over most of the South China Sea.

China has refused to accept that ruling. Its top envoy in Manila said that position is unlikely to change.

Mr Duterte's visit to China is also being shadowed by tensions stoked by the unannounced passage of several Chinese warships through the Philippines in recent months.

Security officials have reported several sightings since February, including a Chinese frigate, and various transport, supply and surveillance vessels sailing near an island in Palawan province in central Philippines, and through the Sibutu Strait, which separates the Philippines' Sulu archipelago and Borneo.

Lieutenant-General Cirilito Sobejana, commander of the military's Western Mindanao Command, said the sail-throughs did not appear to be "innocent", as there was no prior notification and the Chinese ships turned off their automatic identification systems to avoid radar detection.

Brigadier-General Edgard Arevalo, the military spokesman, said the passages "could be considered trespassing".

The foreign ministry this month made diplomatic protests over these incidents.

It has also protested the presence of more than 100 Chinese fishing vessels off Thitu island, a tiny island it holds near China's militarised artificial island at Subi Reef, last month.

Apart from these purported incursions, there have also been flare-ups over the growing population of Chinese workers across Metro Manila.

Last week, Defence Secretary Delfin Lorenzana voiced concerns over the proximity of Chinese offshore gaming companies to military bases across Metro Manila.

He warned that Chinese nationals working for these firms could be mobilised to spy on the Philippines.

Chinese Ambassador Zhao Jianhua said Filipinos working in China could just as easily be accused of spying.

Mr Lorenzana said this was "preposterous".

Mr Duterte himself weighed in, saying China could spy on the Philippine even from afar.