Manila has lodged a diplomatic protest after more than 250 Chinese ships were spotted in recent months off a Philippine-held island in the hotly disputed South China Sea, a presidential spokesman said on Monday.

Under President Rodrigo Duterte the Philippines has largely set aside the once-heated territorial stand-off over Beijing’s claims to the resource-rich waterway in order to court Chinese investment.

However, Duterte spokesman Salvador Panelo told reporters Manila had spoken up after reports emerged that a number of Chinese ships passed near Pag-asa (Thitu) island.

The military put the number of vessels, including fishing and coast guard boats, at 275 for the first three months of this year.

“The DFA [Department of Foreign Affairs] has already made a diplomatic protest over it,” Panelo said. “The mere fact that they are there and just staying there for a week. Why, what are you doing?”

He did not say when the protest was filed and the DFA did not immediately issue any statements.

Control over the South China Sea is a point of regional contention because trillions of dollars of goods traverse through it and rich petroleum reserves are thought to sit deep beneath its waters.

China, the Philippines, Brunei, Malaysia, Taiwan and Vietnam have all staked claims to the sea.

Beijing’s ambassador to Manila Zhao Jianhua told reporters there are Chinese fishermen operating near the island and he pledged China would look into the number of vessels.

“We do not know whether there is militiamen in that area or not, but clearly it is an area that is disputed,” he said.

“We have been handling this issue through friendly diplomatic channels so you don’t need to worry about whether there will be any kind of outbreak of conflict,” he added.

This protest comes as the US and the Philippines kicked off on Monday their largest military exercises, which have been running for decades between Washington and its former colony.

The US has aimed to shore up its relationship with its long-time ally, as Duterte has taken steps toward China in an effort to unlock billions of promised trade and loans.

During a visit to Manila in March, US Secretary of State Mike Pompeo vowed to defend the Philippines from “armed attack” in the South China Sea, a pledge Philippine leaders had long sought.