The Philippines yesterday accused Chinese fishermen of dumping cyanide in waters around a contested atoll in the South China Sea in a bid to loosen Manila’s hold over the feature, drawing a fierce denial from Beijing.
In an interagency press briefing in Manila, Philippine security officials said that they had discovered cyanide on Chinese boats close to Second Thomas Shoal, a submerged reef in the Spratly Islands that was the focus of tense stand-offs between the two nations’ coast guards in 2024 and 2025.
Laboratory tests confirmed the presence of the toxic substance in bottles seized by the Philippine Navy last year at the atoll, which Manila refers to as Ayungin Shoal, the officials said.
“The yellow bottles seized from the sampans contain cyanide, a highly toxic chemical known to cause severe damage to human and maritime systems,” Gen. Cornelio Valencia Jr., the assistant director of the National Security Council, said during the briefing, as per Rappler.
Valencia said that the cyanide could have had serious consequences for marine life in the area and threatened to weaken the integrity of the reef supporting the BRP Sierra Madre, a grounded Philippine Navy warship that serves as Manila’s outpost at the atoll.
Sodium cyanide is sometimes used in fishing, since the chemical stuns the fish without killing it, making it easy to catch in a net, or even by hand. The Philippine authorities have previously accused Chinese fishermen of using cyanide to catch fish at Scarborough Shoal, another disputed shoal in the South China Sea.
In this case, however, Valencia alleged that the use of cyanide had a more nefarious goal. “We wish to underscore that the use of cyanide in Ayungin Shoal is a form of sabotage that seeks to kill local fish populations, depriving Navy personnel of a vital food source,” Valencia added.
The officials said that there are at least four documented instances from February 2025 to March 2026 in which Filipino soldiers who were deployed to the BRP Sierra Madre seized or observed yellow plastic bottles of popular Chinese dishwashing liquid brands. The bottles were carried in small wooden boats that were sent out from Chinese maritime militia vessels, supposedly civilian craft that China uses to bolster its presence in contested parts of the South China Sea. The seized bottles were then tested and found to have contained cyanide.
Rear Adm. Roy Vincent Trinidad, spokesperson of the Philippine Navy, told reporters that no ill health effects had been detected among the personnel stationed aboard the BRP Sierra Madre. Still, he said that “based on the pattern we have uncovered, this is a deliberate attempt not only to destroy the environment but to deprive the men on board of food and water.”
The BRP Sierra Madre is manned by a small contingent of marines who are dependent on both regular supplies of water, fuel, and food from the Navy, supplemented by locally caught fish and desalinated water.
Throughout late 2023 and 2024, the China Coast Guard (CCG) mounted a campaign to disrupt these resupply missions, resulting in a series of tense confrontations between the two sides. After a major clash in June 2024 that injured eight Filipino sailors, one of whom lost a finger, Manila and Beijing came to a “provisional agreement” by which the Philippines could resupply the shoal. Tensions henceforth moved to other disputed features in the South China Sea, including Scarborough Shoal and Sabina Shoal.
The Chinese Foreign Ministry dismissed the Philippines’ assertions yesterday, with spokesperson Guo Jiakun describing them as “utterly unconvincing and not worth refuting.”
“The Philippine side illegally harassed the Chinese fishing boats conducting normal fishing, grabbed the fishermen’s living supplies, and staged this so-called cyanide stunt,” Guo said. “There is no credibility whatsoever to their story.”
Late last month, China and the Philippines held high-level talks last month over the South China Sea for the first time since early 2025. The two sides discussed “initial exchanges on oil and gas cooperation and emphasized stable energy and fertilizer access,” Reuters reported.
The Chinese government has also disputed past Philippine claims about the use of cyanide at Scarborough Shoal, describing them as “totally