MANILA (Reuters) - The Philippines will lodge a protest over the unannounced presence of two Chinese research vessels in its exclusive economic zone (EEZ), one of at least three diplomatic challenges in recent weeks amid a souring of relations.

FILE PHOTO: Philippine President Rodrigo Duterte gestures during his fourth State of the Nation Address at the Philippine Congress in Quezon City, Metro Manila, Philippines July 22, 2019. REUTERS/Eloisa Lopez

The protest comes ahead of President Rodrigo Duterte’s planned visit to Beijing this month, during which he has promised to raise Manila’s South China Sea international arbitration victory over Beijing with leader Xi Jinping, having avoided confronting the thorny issue for three years.

Historically frosty bilateral relations had warmed under Duterte, but he is looking increasingly awkward in defending his controversial approach to China amid constant activity by its coastguard, navy and paramilitary fishing vessels in Philippine-controlled areas of the South China Sea.

“Firing off diplomatic protest,” Foreign Affairs Secretary Teodoro Locsin said in a Tweet on Friday.

The Philippines has protested the presence of more than 100 Chinese fishing vessels off Thitu island, a tiny island it holds near China’s militarized artificial island at Subi Reef.

It also protested the unannounced passage of Chinese warships within the Philippines 12-mile territorial sea, which Defense Secretary Delfin Lorenzana on Friday said had happened on several occasions since February, most recently in July.

The latest protest centers on the lingering presence of two Chinese scientific research boats off the Philippines’ Pacific coast, within the 200 mile EEZ.

Lorenzana told news channel ANC that like other countries conducting research, or moving warships near its coastline, China should have informed the Philippines about it.

“We can always protest to the Chinese government. It’s telling them we know what you are doing and please tell us what you are doing there,” he said.

China’s embassy in Manila did not immediately respond to a request for comment.

Duterte late Thursday called for a quicker creation of a code of conduct between Southeast Asian countries and China for the South China Sea, a conduit for in excess of $3.4 trillion of goods each year.

The Philippines, China, Vietnam, Taiwan, Brunei and Malaysia have overlapping claims of sovereignty within the South China Sea. Via its nine-dash line on maps, China says it has historic jurisdiction to almost the entire sea, although the 2016 arbitral ruling declared that invalid under international law.

On Thursday, Vietnam said a Chinese survey ship embroiled in a tense month-long standoff with Hanoi’s vessels left the Southeast Asian nation’s continental shelf.