Benigno Aquino, President of the Philippines, said that two survey vessels from China had been seen in a disputed gas-rich area of the South China Sea which the Philippines consider their exclusive economic zone, Reuters reports.

China is trying to secure a claim over the entire South China Sea, with Brunei, Malaysia, the Philippines, Taiwan and Vietnam also vying to strengthen their foothold on a lucrative commercial route where USD 5 trillion worth of goods traverse every year.

“Recently, we got a report from the armed forces that there were two hydrographic ships in Recto Bank, about 80 nautical miles off Palawan, clearly within our exclusive economic zone,” Aquino told a television interview aired on Sunday. Recto Bank is also known as Reed Bank.

“What are they doing there? What kind of studies are they conducting? I hope the presence of these ships will not lead to an increasing tension between the two states.”

The two countries had their first clash in the South China sea in 2011, in an incident in which Chinese patrol boats confronted Veritas Voyager survey ship hired by Anglo-Filipino Forum Energy PLC, which won a contract to explore the Reed Bank.

“The frequent passage of Chinese vessels in Recto Bank is not an innocent exercise of freedom of navigation but is actually done as part of a pattern of illegitimate sovereign patrol in the Philippines’ exclusive economic zone,” Charles Jose, Philippine foreign ministry spokesman, said in a statement on Monday.

“This conduct of sovereignty patrols by China is in violation of both UN Convention on the Law of the Sea and the Declaration of Conduct of Parties in the South China Sea (DOC), specifically paragraph 5.”