Officials warn cyanide could harm marine life and weaken the reef supporting a grounded warship.

The Philippines says it has discovered cyanide on Chinese boats near a disputed South China Sea atoll, warning the substance could be used for “sabotage” against local fish populations.

Security officials from the Philippines said on Monday that laboratory tests confirmed the presence of the toxic substance in bottles seized by the Philippine navy last year at Second Thomas Shoal, a submerged reef in the Spratly Islands of the South China Sea.

The officials warned that the cyanide could have had serious consequences for marine life and weakened the reef supporting a warship that Manila grounded on the atoll to reinforce its maritime claim.

“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,” Cornelio Valencia, National Security Council spokesperson, told a news conference, using the Philippines’ name for the atoll.

Valencia added that cyanide could damage the reef and “ultimately compromise” the warship’s stability.

The Foreign Ministry of China rejected the Philippines’ assertions, calling them a “stunt”.

“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. There is no credibility whatsoever to their story,” spokesperson Guo Jiakun said.

The Philippines has accused China of disrupting resupply missions to ⁠troops on the vessel, including a June 17, 2024, incident that ⁠turned violent and resulted in a Filipino sailor losing a finger.

In December 2025, the Philippines accused Chinese coastguard ships of firing water cannon at Filipino fishermen near the disputed Sabina Shoal, injuring three people and causing “significant damage” to two fishing vessels.

China has denied allegations of aggressive conduct during such encounters and accused the Philippines of trespassing in its waters.

China and the Philippines held high-level talks last month over the South China Sea, exploring ⁠preliminary steps towards oil and gas cooperation, and confidence-building ⁠measures at sea, including communication between their coastguards.

The Philippine Foreign Ministry said in a statement on Monday that the scope of the coastguard cooperation would be limited, and “does not contemplate cooperation in sensitive operational areas”, adding that there ‌had been no discussions on joint patrols.

China claims nearly all the South China Sea, including areas claimed by Brunei, Malaysia, the Philippines and Vietnam.

A 2016 ruling by an international ‌arbitral ‌tribunal found Beijing’s sweeping claims had no basis under international law, a decision China rejects.

More than $3 trillion in annual ship-borne commerce travels through the key waterway.