U.S. Secretary of State Antony Blinken and Philippine Foreign Secretary Teodoro Locsin on Thursday expressed "shared concerns" over the massing of Chinese "maritime militia vessels" in the South China Sea, the U.S. State Department said.

The U.S. Navy's 7th Fleet said the same day that its aircraft and ships, such as the nuclear-powered Theodore Roosevelt aircraft carrier, conducted exercises in the South China Sea on Friday local time "in support of a free and open Indo-Pacific."

Photo taken March 7, 2021, shows Chinese fishing vessels near the Spratly Islands in the South China Sea. (Photo courtesy of the Philippine government)(Kyodo)

During their telephone talks, Blinken reaffirmed the applicability of the U.S.-Philippine mutual defense treaty to the sea, in parts of which the Philippines and China have competing ownership claims.

The top diplomats also repeated their calls on China to abide by a 2016 ruling of the Permanent Court of Arbitration that invalidated China's historical claim over much of the South China Sea, according to department spokesman Ned Price.

The Philippines and China have been trading barbs over the massing of Chinese vessels in the area of Whitsun Reef, known in the Philippines as Julian Felipe Reef and in China as Niu'e Jiao, in the disputed Spratly Islands in the South China Sea.

The vessels have been staying there since they were spotted in early March.

Philippine Defense Secretary Delfin Lorenzana has called on the vessels to leave the area, which lies within the Philippines' exclusive economic zone.

But the Chinese Embassy in Manila has said the reef is part of China and that the vessels have recently taken shelter near the reef due to rough sea conditions.

At their talks, Blinken and Locsin also indicated plans to enhance bilateral and multilateral cooperation on South China Sea issues.

Friday's U.S. naval exercises in the sea involved the Theodore Roosevelt Carrier Strike Group and the Makin Island Amphibious Ready Group with members of the 15th Marine Expeditionary Unit aboard the Makin Island amphibious assault ship and two other ships, according to the 7th Fleet.

Two aircraft carrier strike groups from the Navy conducted a joint exercise in February in the South China Sea, in the first such exercise since U.S. President Joe Biden took office in January.

China has rattled the region in recent years through large-scale island-building and base construction activities in the sea.