Japan says recent Chinese military activities in the Pacific -- where Beijing's two operating aircraft carriers were sighted simultaneously for the first time -- reveal its intent to improve operational capacity in remote areas.

Chinese officials have hit back, calling the carrier outings routine training.

Japanese forces must "through warning and surveillance show Japan's will and capability to deter any attempt of changing the status quo by force unilaterally", the defence minister Gen Nakatani told reporters on Friday.

Japan says Chinese fighter jets from the Shandong aircraft carrier flew "unusually close" -- within 45 metres (150 feet) -- to a Japanese military patrol plane on Saturday and Sunday.

Also on Sunday, the Chinese jets cut across airspace in front of the Japanese plane at a distance that the patrol aircraft could reach within seconds, Tokyo says.

On Thursday, Beijing offered its own explanation.

"The root cause of the risk to maritime and air security was the close reconnaissance of China's normal military activities by a Japanese warplane," said foreign ministry spokesman Lin Jian.

"The Chinese side urges the Japanese side to stop this kind of dangerous behaviour," he added.

And on Friday Beijing said it had "already stated" its stance.

"China has also maintained communication with Japan through diplomatic channels and made clear China's solemn position," Lin said.

Nakatani said Friday that "Chinese statements portraying Japan as the cause of the latest incident are unacceptable".

Japanese military planes "never approach another plane as close as 45 metres", he said.