The Philippines’ defence chief on Wednesday said the reported presence of dozens of Chinese vessels in disputed waters in the South China Sea was an “unacceptable” action that violates the country’s sovereignty.

“The president’s order to the department is clear – we will not give up a single square inch of Philippine territory,” Jose Faustino, the officer-in-charge at the Department of National Defense said in a statement.

He added there was “great concern” over the “reported swarming of Chinese vessels in Iroquois Reef and Sabina Shoal in the West Philippine Sea.” Manila refers to the part of the South China Sea that it claims as the West Philippine Sea.

Faustino’s remarks follow a report last week in the Philippine Daily Inquirer in which a Philippine military commander confirmed the presence of Chinese vessels believed to be manned by militias in the reef and shoal since early this year.

“Our lines remain open to dialogue,” Faustino said. “However, we maintain that activities which violate our sovereignty, sovereign rights, and jurisdiction, and undermine the peace and stability of the region, are unacceptable.”

The Philippines had won a landmark arbitration case in 2016, which invalidated Beijing’s expansive claims in the South China Sea where about $3 trillion worth of ship-borne trade passes annually.

The ruling, which China refused to recognise, states that the Philippines has sovereign rights to exploit energy reserves inside its 200-mile Exclusive Economic Zone, where both Iroquois Reef and Sabina Shoal are located.

Iroquois is 127 nautical miles from the Philippine island of Palawan in the disputed waters, which U.S. Vice President Kamala Harris visited last month to reiterate Washington’s defence commitments to Manila and its support for the 2016 arbitration ruling.

There was no immediate comment from the Chinese embassy in Manila. President Ferdinand Marcos Jr will go to Beijing next month for a state visit.