The Philippines said new surveillance images showed that Beijing is stepping up its presence in disputed South China Sea waters, raising the specter of a diplomatic clash.
The Philippine government on Monday summoned the Chinese ambassador to explain why Beijing had sent a flotilla of 10 ships, including dredgers and barges, to a shoal that an international tribunal ruled belonged to Manila. Defense Secretary Delfin Lorenzana said a Philippine military pilot had photographed the flotilla on Scarborough Shoal on Saturday. He said there was no sign of dredging work but the ships' presence implied intent.
"If they try to construct anything in Scarborough, it will have a far-reaching adverse effect on the security situation," Lorenzana said.
A Chinese Foreign Ministry spokeswoman said China "has maintained a Coast Guard patrol and there are some fishing boats undertaking fishing operations in the area. The situation hasn't changed. We hope it won't be hyped up, and that both sides can work together to build trust."
During his two months in office, Philippine President Rodrigo Duterte has generally shunned international diplomatic norms. He has berated the UN secretary-general, international human- rights agencies and the US ambassador to Manila, accusing them of meddling in the country's internal affairs by criticizing his bloody war on drugs and crime.
He often does so using off-color language, such as on Monday when he referred to US President Barack Obama in Tagalog with an expression widely translated to mean "son of a bitch." Obama subsequently canceled a Tuesday meeting with Duterte.
Yet Duterte has so far been more circumspect on China, as he seeks to repair relations that have reached new lows recently over the two countries' territorial rivalry.
The revelation over the flotilla at Scarborough Shoal will test Duterte's approach when he is scheduled this week to meet with Chinese officials and other leaders at the Association of Southeast Asian Nations Summit and the East Asia Summit in Laos.
"Why is China treating us this way?” said Duterte, according to Agriculture Secretary Manny Piñol, who attended a cabinet security briefing at which Duterte was shown the surveillance photos. In a Facebook post on Sunday, Piñol said Duterte was upset to learn that China seemed determined to provoke the Philippines, despite his conciliatory efforts.
Beijing has rejected the tribunal's ruling but made overtures to Duterte, inviting him to reset bilateral relations and open negotiations. He reciprocated in August by dispatching former President Fidel Ramos to Hong Kong to meet senior Chinese officials to prepare the ground for formal talks later this year.
On Tuesday, Shinzo Abe, Japan's prime minister, and Duterte struck a deal for Tokyo to provide Manila with two large patrol vessels and loan it up to five used TC-90 surveillance planes.
The deal marks a step in attempts by Japan and other powers to offset China's growing strength by creating a chain of regional allies committed to the status quo and the rule of law at sea.
The agreement comes as countries including India and the US have increased security cooperation with Southeast Asian nations with whom they share common cause in checking China's maritime ambitions.
Coastguard vessels are often on the front line of territorial disputes in Asia, entering disputed waters where a naval ship would be too provocative. There has been a trend towards ever bigger coastguard vessels that can intimidate rivals with their sheer size.
Although Japan has no direct stake in the South China Sea, much of its trade passes through the area. It also has its own territorial dispute with Beijing over the East China Sea islands known as the Senkaku in Japan and the Diaoyu in China.
There have been a series of stand-offs in recent weeks between Japan and Chinese fishing and coastguard vessels around the islands. By supporting the Philippines and other regional countries, including Vietnam, Tokyo hopes to maintain a united front against changes in the territorial status quo and thus strengthen its own security.
On Tuesday, Philippine Foreign Minister Perfecto Yasay said China will be the "loser" if it does not recognize the Hague-based court ruling against its territorial claims in the South China Sea.
"We are trying to make China understand especially when the dust settles that unless they respect and recognize the abitral tribunal, they will be the losers at the end of that day on this matter," Yasay told a congressional hearing.
Prior to starting bilateral talks, the Philippines plans to seal a deal for China to allow Philippine fishermen to access the resource-rich waters, Yasay said.
China seized Scarborough Shoal in 2012, denying Philippine fishermen access, one of the factors that prompted Manila to seek arbitration.
"When we start formal negotiations or bilateral engagements with China, we will have to do it within the context of the arbitral decision. There are no buts or ifs insofar as our policy on this matter is concerned," Yasay said.
Duterte said last week he expects talks with China to start within a year.