MANILA (Reuters) - The Philippine defense minister on Thursday said he was “disturbed” by what he believes are survey missions by Chinese ships deep into its 200 nautical mile exclusive economic zone (EEZ) and in an area designated as its continental shelf.

Philippine Defence Secretary Delfin Lorenzana answer questions during a Reuters interview at the military headquarters of Camp Aquinaldo in Quezon city, metro Manila, Philippines February 9, 2017. REUTERS/Romeo Ranoco

Defense Secretary Delfin Lorenzana said Chinese ships had been monitored in recent months at various locations close to the Philippines. A warship was detected 70 miles off its Western coast in the South China Sea and survey ships were seen at the north and south of the eastern seaboard.

While President Rodrigo Duterte has frequently praised China amid a warming relationship, Lorenzana has remained openly suspicious, noting that its fortification of manmade islands inside the Philippine EEZ has continued.

China claims most of the energy-rich waters through which about $5 trillion in ship-borne trade passes every year. Neighbors Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have claims.

Lorenzana said that satellite imagery provided by allies had tracked Chinese vessels for three months last year in Benham Rise, a vast area declared by the United Nations as part of the Philippines’ continental shelf.

“I am disturbed by China’s presence there, it is annoying if they will claim the area,” he told Reuters.

Lorenzana earlier gave a presentation to media showing where Chinese vessels had been and said he was suspicious of its activities to the east, because China had never laid claim to those waters.

He told reporters he had received information suggesting China may have been surveying water depths to prepare submarine routes to the Pacific. He has told the navy to intercept vessels if they return.

The reported Chinese activity comes as the two countries seek to forge closer trade and investment ties under Duterte after years of bickering and mistrust, mostly over the South China Sea.

The Philippine finance ministry issued a statement on Thursday lauding the two countries’ moves to fast-track big infrastructure projects to completion within three years.

The Philippines won an international arbitration award last year that invalidated China’s claims to sovereignty over almost the entire South China Sea. Instead of pressing China to comply, Duterte has chosen to tap Beijing for business, and has promised to deal with the maritime dispute later.

Lorenzana voiced frustration on Thursday that since Duterte took office eight months ago, the Philippines had submitted about a dozen requests to the Chinese embassy to explain its maritime activities, but each time it had denied they had taken place.

Asked about activities of the survey ships Lorenzana had referred to, China’s Foreign Ministry spokesman Geng Shuang said he was unaware of the report.