Japan's annual defense paper says an increasingly bold China is attempting to change the regional status quo with shows of force.
The paper adopted Tuesday is the first since Japan nationalized a set of disputed islands in September, worsening a long-standing territorial dispute with China.
Since then, China has sent regular patrol boats and aircraft near the area in what is seen as an attempt to establish defacto control of the Japanese-administered islands.
The Japanese report called such actions "dangerous" and "extremely regrettable," warning it could cause a "contingency" in the future.
In particular, the paper criticized China for a January incident in which Tokyo said a Chinese navy boat locked its pre-firing radar on a Japanese destroyer. China has denied the claim, calling it inaccurate and provocative.
On Tuesday, China's foreign ministry slammed the Japanese accusations as unfounded. It also returned the accusation, saying "some political forces" in Japan are "advocating for the strengthening of their military and preparing for war."
Prime Minister Shinzo Abe came to power in December. He has taken a firm stance on the island issue and is seeking changes to the country's post-World War Two pacifist constitution in order to legitimize the military.
The islands have been the focus of a decades-long dispute between the Asian powers. But the situation worsened in September, when Japan purchased three of the five islands from their private Japanese owner.
Besides the increased patrols, the move also set off days of angry anti-Japan protests across China, sending relations plummeting to the lowest level in years.
The uninhabited islands, known as Senkaku in Japan and Diaoyu in China, are surrounded by rich fishing grounds and possibly by energy deposits.