Japan Maritime Self-Defense Force's PC3 surveillance plane flies around the disputed islands in the East China Sea, known as the Senkaku isles in Japan and Diaoyu in China, in this October 13, 2011 file photo.
Japan Maritime Self-Defense Force's PC3 surveillance plane flies around the disputed islands in the East China Sea, known as the Senkaku isles in Japan and Diaoyu in China, in this October 13, 2011 file photo.Reuters

Since Japanese Prime Minister Shinzo Abe's infamous visit to the Yasukuni Shrine, China has been turning more bitter in its aggression. Further fueling tensions, three Chinese government ships on Sunday morning briefly entered what Japan sees as its territorial waters, near the disputed islands (controlled by Japan but also claimed by China).

This was the latest in such occasional entries made into the area by Chinese ships. In a strong reply, Japan's defense minister Itsunori Onodera vowed on Sunday to defend the country's territory.

It's the second time of late that Chinese ships have been spotted in the area by Japanese coast Guards. The last was on 29 December, when three Chinese coastguard ships entered the disputed waters off the Tokyo-controlled islands in the East China Sea.

As per reports in the local media, the Chinese coastguard vessels sailed into the 12-nautical-mile territorial waters at about 8.30am on Sunday (11.30pm GMT Saturday) off one of the Senkaku islands, which China also claims and calls the Diaoyus.

As per reports by the Japan coastguard, the enemy vessel stayed in the Japanese waters for about two hours. However, there have been no reports of any confrontation by either country.

Following the incident, Japan on Sunday criticized China's new fishing restrictions in the South China Sea. "I'm afraid not only Japan but the international society as a whole has a concern that China is unilaterally threatening the existing international order" with its new restrictions in the South China Sea and the creation of an air defence identification zone, Japanese defense minister Onodera said.

The new fishing rules, which Washington called as "provocative and potentially dangerous", took effect on 1 January and require foreign fishing vessels to obtain permission to enter disputed waters in the South China Sea, which Beijing says is under its jurisdiction.

The situation over the inlet remains tense, as on previous occasions the Japanese have tried to chase the Chinese ships away, which is potentially risky and may trigger an armed conflict.

Since December, there has always been one incident after the other in the area. On 5 December, a Chinese vessel forced the USS Cowpens, which was operating in international waters, to maneuver away in order to avoid hitting the PLA Navy ship, which was on a collision course.

In recent months, tensions have soared after Beijing announced the air defense identification zone in the East China Sea, covering large parts of the disputed isles. Following this, the announcement by Japan to double its defense expenditures, triggered much aggressive threats from China. All this only flared more after Japanese PMo Abe visited the controversial Tokyo shrine, seen by critics as a symbol of Japan's wartime aggression.