By Agence France-Presse
Defence Minister Itsunori Onodera said China had been "unilaterally escalating" its military activities in the past year, including carrying out new airborne operations around Japan and running a nuclear submarine near disputed East Coast isles.
"China has been rapidly improving its military strength and fast increasing its military activities," Onodera told an annual gathering of the top brass of Japan's Self-Defense Forces.
"It is unilaterally escalating its military activities in the sea and aviation spaces around our country. This has become a significant concern for our country's defence," he said.
Onodera made the remarks as Tokyo attempts to improve its tense diplomatic ties with Beijing, with Prime Minister Shinzo Abe expected to visit Japan's biggest trade partner next month.
On coming to power, Abe took a firm position on Japan's claims to a disputed island chain in the East China Sea, aggravating tensions with Beijing.
But he has since softened his rhetoric, and called on China to press North Korea to give up its nuclear and missile programmes.
Japanese businesses have also voiced desire for closer ties with China to boost trade.
Onodera said Russia was also flexing its military muscle.
Moscow is planning to hold its biggest drill since the Cold War era and is bringing powerful weapons, including ground-to-air missiles, to the disputed Southern Kuril islands, he said.
"We are seeing movement to again push forward its military activities in the Far East," he said.
Onodera also repeated that North Korea continues to pose a "serious and imminent threat" to Japan, despite international diplomacy intended to convince Pyongyang to denuclearise.