Chinese government ships were spotted in the contiguous zone off the Senkaku Islands in Okinawa Prefecture, the 11th Regional Coast Guard Headquarters announced Monday.
It marks the 283rd day this year that the sighting of a Chinese government vessel in the zone has been confirmed, surpassing last year's record, since the nationalization of the islands in September 2012.
The government is becoming increasingly wary of China's moves.
According to the coast guard headquarters in Naha, a ship belonging to the China Coast Guard were seen sailing in the zone about 31 kilometers northwest of the Senkaku chain's Kubashima island as of 3 p.m. Monday.
A Japan Coast Guard patrol ship continued exercising vigilance against the Chinese ships to make sure they did not enter territorial waters.
Contiguous zones extend a further 12 nautical miles (about 22 kilometers) from territorial waters, which are the area 12 nautical miles from the baseline of the coast. Although foreign vessels are allowed to sail in the zones as if in international waters, the coastal state is given certain authority such as exercising measures to prevent criminal activities.
Chinese government vessels have been increasingly active in waters off the Senkaku Islands.
In the April-August period this year, they sailed in the contiguous zone for 111 consecutive days.
In October, they remained in the territorial waters for 57 hours 39 minutes, the longest duration since the nationalization of the islands. They even approached a Japanese fishing vessel in operation.
"We have taken it as an extremely serious matter that there have been a number of incidents of navigation in the contiguous zone and intrusion into our territorial waters," Chief Cabinet Secretary Katsunobu Kato said at a press conference Monday.
Tokyo has made stern protests to Beijing through diplomatic channels on each occasion, but there is no sign that China is backing down on its activities.
A Japanese government source described the situation as China "trying to turn its claims over its territorial rights into a fait accompli."
"We will heighten our vigilance and make every effort to collect information through coordination among the relevant ministries and agencies in order to keep an eye on the situation," Kato said. "We will continue to respond to China with a firm, stern attitude."
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