Beijing has called sailings by its warships through the Tokara Strait off Kagoshima Prefecture "legal" under international law, days after Tokyo said a Chinese military survey vessel had entered Japan's territorial waters in the area.

The sailing was part of a series of recent moves by the Chinese military near Japan that Tokyo has said are “cause for concern.”

China’s Foreign Ministry on Monday called the nearly 200-kilometer strait one used “for international navigation,” according to the United Nations Convention on the Law of the Sea (UNCLOS).

“Chinese ships passing through the strait are exercising the right of transit passage, which is completely legitimate and legal,” ministry spokeswoman Mao Ning told a news conference.

Japanese Defense Minister Minoru Kihara said Tuesday that Tokyo had lodged a protest with Beijing over the incident Saturday, noting the high frequency of Chinese naval vessels entering Japanese territorial waters, as well as the first recorded instance of a Chinese military plane incursion into Japanese airspace last week.

“It’s difficult to say with certainty the purpose or intention of the Chinese naval vessels' voyage, including its relevance to the incident of the Chinese military aircraft violating Japanese airspace, but China has been increasing and expanding its military activities in the vicinity of our country in recent years,” Kihara told a news conference.

Saturday’s incident saw a Chinese Shupang-class survey ship enter the territorial waters southwest of Kagoshima’s Kuchinoerabu Island for about two hours, according to the Defense Ministry in Tokyo. It was the 13th time since September last year that a Chinese military vessel had entered Japanese territorial waters, the ministry said.

Just five days earlier, on Aug. 26, the Air Self-Defense Force scrambled fighter jets as a Chinese military Y-9 intelligence-gathering plane approached and briefly entered Japanese airspace near the Danjo Islands off Nagasaki Prefecture for two minutes.

Tokyo strongly protested the Chinese military aircraft’s incursion as “utterly unacceptable,” calling the move a threat to Japan’s safety. Chinese officials last week denied any intention to intrude into Japan’s airspace, and have denied any link between the flight and sailing, which came just five days apart.

“Regarding Japan's claim that Chinese military aircraft entered Japanese airspace, China responded last week and clarified its position,” Mao said. “I would like to emphasize that there is no need to deliberately associate or overinterpret.”

Territorial airspace refers to the skies above a territory and its territorial waters 12 nautical miles (about 22 km) from the coastline. Flying freely inside the territorial airspace of another country is not permitted under international law and has in the past led to shootdowns.

UNCLOS, meanwhile, allows for vessels — including warships — the right of “innocent passage,” which lets them pass through the territorial waters of other states so long as they do not engage in activities that would endanger the “peace, good order or security” of the coastal state.

But survey vessels can probe the topography, depth and temperature of the seabed, and, according to Japanese Defense Ministry officials, the Chinese military may be collecting such information to help with the operations of their submarines in the Tokara Strait, a key entryway into the Pacific Ocean for the Chinese navy.

Sailing under the premise of innocent passage, some say, could constitute a violation of UNCLOS, since doing so while collecting survey information for submarine operations could endanger a country’s security.

China, however, has emphasized that it had exercised the right of “transit passage” in Saturday’s sailing. Transit passage, which has fewer restrictions than innocent passage, generally means passage through a strait or exclusive economic zone “solely for the purpose of continuous and expeditious transit.”

During a transit passage, foreign ships — including marine scientific research and hydrographic survey vessels like the Chinese ship that sailed through the Tokara Strait — “may not carry out any research or survey activities without the prior authorization of the states bordering straits,” according to UNCLOS.