The Philippines has released pictures purportedly showing Chinese boats near a disputed shoal in the South China Sea.
The photos were shown just hours before Southeast Asian nations were due to meet China's premier at a summit in Laos on Wednesday.
China this week insisted it had not launched any efforts to begin construction at the shoal, which has huge strategic importance for its 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," Arsenio Andolong, defence department spokesman, told AFP news agency.
"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.
Major source of tension
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 230km 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.