Chinese coast guard vessels have sprayed water cannon on Filipino fishermen in the disputed South China Sea, while a Philippine military plane circled the area.
The incident on Thursday took place near the Scarborough Shoal, which China seized from the Philippines in 2012.
Video uploaded to social media shows sea water shooting up into the air, and scores of Filipino fishermen running away.
The Philippines earlier said it had sent military jets to patrol the Scarborough Shoal.
China recently asked the UN tribunal reviewing its claims in the area not to hold a hearing.
In recent years, tensions have increased in the South China Sea, where China has built islands with airstrips, runways and weaponry.
It has also placed undersea observation posts and islets in the region, which a US diplomatic cable quoted as describing as the “Pearl Harbour of the 21st century”.
The Philippines accused China of reopening an island on the reef earlier this month, prompting the government to send an air force jet fighter to inspect the area.
Two weeks later, the coast guard vessels returned to the reef, and the Philippines said that its planes had been doing flyovers as a show of support.
“This was confirmed by the foreign ministry that we have sent our planes to patrol the area,” a Philippine military spokesman told the AFP news agency.
It was not immediately clear whether the incident took place on Thursday.
More than 50 countries are involved in disputes with China in the South China Sea, according to the US, including countries that are the target of its “freedom of navigation” patrols.