The Philippines released photos Wednesday to back its claims that China had secretly begun work to cement control over a crucial shoal in the hotly contested South China Sea.
The release of the images, which claimed to show Chinese ships preparing to build an artificial island on Scarborough Shoal, came hours ahead of a meeting between Southeast Asian leaders and Chinese Premier Li Keqiang in Laos.
China this week insisted it had not launched any efforts to begin construction at the shoal, which has enormous strategic importance for Beijing's ambitions to control the sea and weaken US military influence in the region.
But the Philippines said, the images showed Chinese ships at the shoal last weekend that were capable of dredging sand and other activities required to build an artificial island.
"We have reason to believe that their presence is a precursor to building activities on the shoal," defence department spokesman Arsenio Andolong told AFP in a text message.
"We are continuing our surveillance and monitoring of their presence and activities, which are disturbing."
China claims nearly all of the sea, through which $5 trillion in shipping trade passes annually, even waters approaching the coasts of the Philippines and other Southeast Asian nations.
The competing territorial claims have long been a major source of tension in the region, with China using deadly force twice to seize control of islands from Vietnam.
Concerns have escalated sharply in recent years as China has built artificial islands on reefs and islets in the Spratlys archipelago -- another strategically important location -- that are capable of supporting military operations.
An artificial island at Scarborough Shoal would potentially give China a military base close to where US forces regularly operate on the Philippine main island of Luzon, which is just 230 kilometres (140 miles) away.
Philippine President Rodrigo Duterte had said, he did not want to anger China by highlighting the territorial row at the summit of regional leaders in Laos this week.
But the release of the photos came just a few hours before Duterte and other leaders from the 10-member Association of Southeast Asian Nations met China's Li.
A UN-backed tribunal ruled in July that China's claims to almost all of the sea had no legal basis and its construction of artificial islands in disputed waters was illegal.
China has vowed to ignore the ruling.
US President Barack Obama is also in Laos for the regional meetings, which will conclude on Thursday with an East Asia summit.