The protest was formally lodged by Vice Foreign Minister Shinsuke Sugiyama, reported the Japanese Ministry of Foreign Affairs. The incidents in question happened on Wednesday and Thursday when a pair of vessels — a frigate and a submarine — came within miles of the island chain.
While the identity of the submarine was never confirmed, it is believed to be a PLAN vessel. The submarine entered and exited the disputed area several times, while the frigate- which was identified as Chinese- passed through once.
Sugiyama expressed "great concern" over the naval action and urged China not to undo efforts to improve relations after years of stressed ties. He particularly took umbrage with the submarine, calling it an "unilateral escalation in new form."
The Defense Ministry of Japan also expressed their concern, although in more vigorous terms. "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 on Friday.
China defended the move on Thursday, calling the islands their sovereign territory and saying the incident was brought on by Japanese naval provocation.
"The [Diaoyu Islands] are a natural part of Chinese territory," Lu Kang, Beijing's Foreign Ministry spokesman, in a Thursday statement. "China's decision to guard the sovereignty of the [Diaoyu] territory is unshaken."
The disputed islands in the East China Sea are known by several names. The earliest known source, a 15th century Chinese sea chart, names them the Diaoyu. A 19th century Imperial Japanese naval record is the origin of their Japanese name, the Senkaku.
Regardless of what name one uses, the disputed islands consist of five islets and three rocks, the largest of which is less than two square miles in area. The uninhabited islands were historically part of China before being annexed by Japan following their victory during the First Sino-Japanese War in 1895. After World War II, the islands were occupied by the United States.
The islands were returned to Japanese control in 1972, but by then a potential treasure trove of oil and natural gas had been identified in the surrounding area. Since then, both China and Taiwan have declared ownership of the islands. China in particular has objected to Japanese activity, including nationalizing several of the islands in 2012 and building a lighthouse on one in 2014.
The US has supported the Japanese claim in the past. In 2014, President Barack Obama pledged to aid Japan if China used military force to stake their claim to the islands. Similarly, President Donald Trump declared US support for the Japanese claim in February.
China has continued to sail naval vessels through the disputed waters, with incidents like the one from Wednesday and Thursday occurring 114 times in 2017 according to the Japanese Coast Guard.