CAMP FOSTER, Okinawa — Japan has complained to China after a pair of the country’s vessels sailed this week near disputed islands in the East China Sea.

Vice Foreign Minister Shinsuke Sugiyama lodged protests Thursday and Friday to China’s ambassador to Japan, a Ministry of Foreign Affairs statement said. The incursions happened Wednesday and Thursday in Japan’s contiguous zone around Okinawa and the Senkaku Islands by a Chinese frigate and submarine presumed to be from China, Japanese officials said.

Sugiyama expressed “great concern” over the provocations and strongly urged China not to hinder efforts to improve the countries’ relationship. He also called sending a submarine to the area a "unilateral escalation in new form" and urged China not to do it again.

The Senkakus — an uninhabited chain surrounded by resource-rich waters between Okinawa and Taiwan — are claimed by Japan, Taiwan and China, which refers to them as Diaoyu.

The United States has long declined to take a position on the islands’ sovereignty; however, Presidents Barack Obama and Donald Trump have both declared that the Senkakus will be protected under the U.S-Japan security alliance.

“Senkaku is Japan’s territory and territorial waters internationally and historically and Japan’s claim about the contiguous zone is absolutely correct,” Defense Minister Itsunori Onodera told reporters Friday morning.

The Japanese military first observed a submarine of unknown origin entering the contiguous zone just outside its territorial waters northeast of Miyako Island on Wednesday evening, a Ministry of Defense statement said.

The submarine’s presence was confirmed by a Japan Maritime Self-Defense Force escort ship and a P-3C aircraft, the statement said. The sub exited the contiguous zone and headed toward the Senkakus on Thursday morning. It then re-entered the contiguous zone around Taisho Island, which is part of the Senkakus, before leaving that afternoon.

A Chinese navy frigate was spotted Thursday morning entering the contiguous zone northeast of Taisho island, the statement said. It left Thursday afternoon.

Japanese Prime Minister Shinzo Abe responded to the incidents Thursday by giving instructions to prepare for all possible contingencies and to coordinate closely with the U.S., according to a statement released by his Cabinet.

Chief Cabinet Secretary Yoshihide Suga told reporters Thursday that Japan will “definitely defend our territory, territorial waters and airspace.” A defense ministry spokesman declined to discuss the measures being taken.

Chinese officials defended the move Thursday, saying that the Senkakus are China’s territory and that the incident was instigated by two Japanese navy vessels.

“The [Diaoyu Islands] are a natural part of Chinese territory,” Lu Kang, Beijing’s foreign ministry spokesman, said Thursday, according to UPI. “China’s decision to guard the sovereignty of the [Diaoyu] territory is unshaken.”

Maritime confrontations and jet interceptions between China and Japan have become commonplace in recent years, especially around the Senkakus. Five submarines have been spotted entering Japan’s contiguous zone since 2013, a defense ministry spokesman said. A Chinese frigate was last seen in the contiguous zone in June 2016.

The Japan Coast Guard said Chinese ships entered Japanese waters 114 times last year.

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