Japan said it was monitoring waters near the disputed Senkaku Islands in the East China Sea after a Chinese naval intelligence ship was spotted in the area. The ship reportedly moved back and forth but never breached Japan’s 12 nautical mile territorial waters, the ministry said, Agence France-Presse reported.

Japanese Defense Minister Gen Nakatani characterized the ship’s movement as “unusual” during a press conference Friday, adding the ministry would continue monitoring China’s navy. Japan planned to strengthen its border security in response to the sighting, a Japanese daily said.

The Dongdiao-class intelligence vessel reportedly is armed with one 37 mm and two dual 14.4 mm cannons, the Japanese ministry said. The Chinese government said the ship’s operations were standard.

Spokesman Hong Lei told a regular briefing the "Chinese naval vessel is conducting normal activities," AFP reported. "It is in line with international law. There is nothing disputable about that," he said.

Tensions between Japan and China have been high over the uninhabited islands in recent years. The Chinese coast guard has repeatedly approached them in a move likely meant to test Japan’s response. Relations between the two regional powers hit a low in September 2012 after the Japanese government sought to nationalize some of the eight islands. The islands are strategically significant as they lie near important shipping lanes, offer rich fishing opportunities and sit near potential oil and gas reserves, the BBC reported. 

While political relations between Japan and China have remained tense, the two countries have stepped up economic cooperation in recent years. Earlier this month, the leaders of South Korea, Japan and China met at their first joint meeting in more than three years to discuss working toward greater economic integration. They also discussed easing regional tensions. 

Nakatani declined to comment on the ship’s aims, but said the Chinese military has boosted activities in the area. He said he expects China "will try to expand the area of its activities further in the future."