The number of Chinese fishing boats in disputed South China Sea waters has increased, according to the Philippine military, in a “alarming” way that threatens the safety of the oil- and gas-rich Reed Bank.

According to the military’s Western Command (WESCOM), there were 47 Chinese fishing boats “swarming” in Iroquois reef, just south of the Reed Bank, as of last month, up from only a dozen in February.

“China must cease its swarming of vessels to respect our sovereign rights,” Ariel Coloma, spokesperson for the Western Command, said in a statement.

There was no immediate comment from the Chinese embassy in Manila.

The Philippines won a landmark arbitration case in 2016 that invalidated China’s expansive claims in the South China Sea, where about $3 trillion worth of sea-borne goods pass every year.

The ruling, which treaty ally United States backs and China refuses to recognise, clarified Philippine sovereign rights in its 200-mile exclusive economic zone (EEZ) to access offshore oil and gas fields, including the Reed Bank, where a natural gas exploration project operated by Philippine firm PXP Energy Corp has been stalled.

Philippine military flights had also recorded the presence of three China Coast Guard ships and two Chinese navy vessels “regularly loitering” at Sabina Shoal, which like Iroquois, is inside the Philippine EEZ.

“These developments raise alarming concerns about China’s intentions and actions within these disputed waters,” WESCOM said.

U.S. Defense Secretary Lloyd Austin told his Philippine counterpart, Gilbert Teodoro, on Thursday that the U.S. commitment to the defense of its ally was “ironclad,” including in the South China Sea, according to a U.S. summary of the call.

The Philippines on Wednesday accused the China Coast Guard of harassment, obstruction and “dangerous manoeuvres” against its vessels, after another incident near a strategic feature of the South China Sea.