Vietnam’s state-owned oil and gas company accused Chinese boats of sabotaging one of its survey ships as tensions in the potentially resource-rich South China Sea escalate.

PetroVietnam said two Chinese fishing vessels cut across seismic exploration cables being laid by the Binh Minh 2 survey vessel early Friday morning. Senior PetroVietnam offical Pham Viet Dung described the move as a “brazen violation” of Vietnam’s sovereignty.

“PetroVietnam vehemently protests the Chinese fishing boats’ action against the Binh Minh 2,” the official said in a statement on the company’s website, urging the Vietnamese government to lodge a protest with Beijing. “We ask that China educate its citizens to respect Vietnamese waters”.

It is the second time that Chinese vessels have reportedly damaged Vietnamese exploration cables, following an incident in May, AP reports.

Southeast Asian nations raised concerns over China’s increasing assertiveness in claiming territories coupled with military expansion and economic growth. Beijing recently stirred already-troubled waters with new Chinese passports claiming most of the South China Sea, provoking anger from its neighboring nations.

The Philippines described the passport maps as “an excessive declaration of maritime space in violation of international law” and refused to stamp the passports.