Verbal skirmishing between Asia's two biggest economies, who dispute ownership of an island chain, escalated as Beijing warned Tokyo that any hostile action in the skies against Chinese drones would be construed as an "act of war".
"We will express our intention as a state not to tolerate a change in the status quo by force. We must conduct all sorts of activities such as surveillance and intelligence for that purpose," Prime Minister Shinzo Abe said in an address to the military.
"The security environment surrounding Japan is becoming increasingly severe. This is the reality," he said. "You will have to completely rid yourselves of the conventional notion that just the existence of a defence force could act as a deterrent."
Abe presided over an inspection of the military at which a US amphibious assault vehicle was displayed for the first time, an apparent sign of Japan's intention to strengthen its ability to protect remote islands.
The defence ministry plans to create a special amphibious unit to protect the southern islands and retake them in case of an invasion.
"There are concerns that China is attempting to change the status quo by force, rather than by rule of law," Abe earlier told the Wall Street Journal in an interview following a series of summits this month with regional leaders.
"But if China opts to take that path, then it won't be able to emerge peacefully," he said in the interview published Saturday.
"So it shouldn't take that path, and many nations expect Japan to strongly express that view. And they hope that as a result, China will take responsible action in the international community," Abe added.
On Sunday Jiji Press and Kyodo News reported that Japan had deployed jets for two days running in response to four Chinese military aircraft flying over international waters near the Okinawa island chain.
Two Y8 early-warning aircraft and two H6 bombers flew from the East China Sea to the Pacific Ocean and back again but did not violate Japan's airspace, the reports said.
The Japanese defence ministry was not immediately available for confirmation.
Japan's military is on increased alert as Tokyo and Beijing pursue a war of words over the disputed islands in the East China Sea that lie between Okinawa and Taiwan.
On Saturday China responded angrily after a report said Japan had drafted plans to shoot down foreign drones that encroach on its airspace if warnings to leave are ignored.
Tokyo drew up the proposals after a Chinese military drone entered Japan's air defence identification zone near the disputed islands in the East China Sea last month, Kyodo said.
"We would advise relevant parties not to underestimate the Chinese military's staunch resolve to safeguard China's national territorial sovereignty," China's defence ministry spokesman Geng Yansheng said in comments posted on the ministry's website.
"If Japan takes enforcement measures such as shooting down aircraft, as it says it will, that would constitute a serious provocation, an act of war of sorts, and we would have to take firm countermeasures, and all consequences would be the responsibility of the side that caused the provocation."
Tokyo and Beijing both claim the small uninhabited islands in the East China Sea. Japan administers them and calls them the Senkakus. China refers to the islands as the Diaoyus.
One of Abe's first decisions as prime minister was to increase the defence budget for the first time in 11 years.
Tokyo also plans to hold a major air and sea exercise next month to bolster its ability to protect its remote islands.
In the Wall Street Journal interview, Abe said Japan had become too inward-looking over the past 15 years, but as it regains economic strength "we'd like to contribute more to making the world a better place".
The Journal said he made it clear that one way Japan would "contribute" would be countering China in Asia.