Philippine fishing authorities said Tuesday they had detained 25 Chinese crew of two seized fishing boats on suspicion of poaching, a move that could further strain ties between two countries at odds over maritime sovereignty.

A joint team from the Philippine coast guard and the Bureau of Fisheries and Aquatic Resources (BFAR) captured the Chinese vessels Monday sailing without permits between waters off Babuyan Island and Batanes province in the northern Philippines.

The Chinese vessels were in waters that are not part of the disputed South China Sea, which China claims almost entirely. Brunei, Malaysia, the Philippines, Vietnam and Taiwan also have conflicting claims.

Tension between the Philippines and China has risen as an international tribunal at the Hague prepares to deliver a ruling in the next few months in a case lodged by Manila in 2013.

The Philippines is seeking a clarification of United Nations maritime laws that could undermine China’s claims to 90 percent of the South China Sea. China has rejected the court’s authority.

The fishing boats were flying an inverted Philippine flag when apprehended Monday, BFAR said in a statement.

Speaking at a regular news briefing in Beijing, China’s Foreign Ministry spokesman Hong Lei said he had no information and needed to further understand the situation.

Documents seized by Philippine authorities showed the vessels were Chinese registered. The fishermen failed to show permits required to enter Philippine waters, BFAR said.

“Based on existing rules, the fact that both foreign fishing vessels were flying a Philippine flag gave rise to the presumption that they are engaged in poaching,” BFAR chief Asis Perez said.

A Philippine court fined nine Chinese fishermen $102,000 each in late 2014 after they were caught with hundreds of sea turtles in the disputed Half Moon Shoal in the Spratly Islands of the South China Sea.

At the time, China protested against the arrests and refused to recognize their trial.