Foreign Secretary Teodoro Locsin Jr. said China’s People’s Liberation Army has been staging exercises off the Paracel Islands since July 1 and Chinese maritime officials have prohibited all vessels from navigating within the area of the maneuvers.

After checking the coordinates of the Chinese-declared no-entry zone where the maneuvers were being staged, Locsin said the waters off the Paracels, which are also claimed by Vietnam, “do not impinge on Philippine territory” but it was still a concern.

“Should the exercises spill over to Philippine territory, then China is forewarned that it will be met with the severest response, diplomatic and whatever else is appropriate,” Locsin said in a statement, without elaborating.

The Philippine warning to China over their territorial conflicts is the strongest so far this year and comes despite an improvement in relations since President Rodrigo Duterte took office in 2016.

Philippine Defense Secretary Delfin Lorenzana also raised alarm on Thursday over the Chinese military exercises. “If they are doing it in the contested areas, that will sound the alarm bells for all the claimants here in the South China Sea,” Lorenzana told an online news forum.

Vietnam protested in April after a Chinese coast guard ship rammed and sank a boat with eight fishermen off the Paracel Islands. The Philippines backed Vietnam and protested two new territorial districts announced by China in large swaths of the sea, adding that China’s assertive actions were taking place while the region was intensely preoccupied with the coronavirus pandemic.

Locsin said those territorial districts in the disputed waters were “null and void” because they were devoid of basis in international law. China has claimed virtually the entire waterway, one of the world’s busiest, on historical grounds but its claim was invalidated by an international tribunal in a 2016 ruling based on the United Nations Convention on the Law of the Sea.

“This is the problem with playing fast and loose with historical narratives and historical names,” Locsin said. “They open themselves to error; unless the real purpose is to excuse unchallenged mistakes that may over time harden into rights.”

China, like any other power, can invoke freedom of navigation while carrying out military exercises, Locsin said, but added that such passages should be done in a straight and uninterrupted voyage.

“We continue to look to China, as our nearest and biggest trading partner,” Locsin said, adding that its participation is essential to the success of any post-coronavirus economic recovery.

But he called on “the erring parties to refrain from escalating tension and abide by the responsibilities under international law” and exercise self-restraint in taking actions that could escalate disputes, especially during the pandemic.