The comments emerged Tuesday after Shinzo Abe proclaimed Japan's powerful military had the right to go into battle in defence of allies, so-called "collective self-defence", in a highly contentious change in the nation's pacifist stance.

The conservative premier, who has long cherished a desire to beef up Japan's armed forces, faced massive opposition from a population deeply wedded to the principle of pacifism that underpins its identity.

He had sought in public to play down the shift, which he said was a necessary update to better protect Japan in a region dominated by an increasingly assertive China and worried by an erratic North Korea, which Wednesday lobbed rockets into the Sea of Japan (East Sea).

But talking to senior officials of his Liberal Democratic Party (LDP) he said "collective self-defence is as significant as the Meiji Restoration", Jiji Press reported Tuesday, without citing sources.

The 1868 Meiji Restoration marks the beginning of modern Japan, when it cast off more than two centuries of feudalism under samurai warriors in which foreign travel was banned and the ports were closed to outsiders.

It saw the emperor return to pre-eminence at the pinnacle of the state and heralded the coming of rapid industrialisation that would lead to the ultimately-thwarted imperial ambitions and the disaster of World War II.

Asked by AFP to expand on the prime minister's comparison, deputy chief cabinet secretary Katsunobu Kato demurred, but did not deny it had been made.

"I decline to comment on it... as the comment was not made in a public arena nor was recorded," he said.

"However the prime minister has said on various occasions, including at the press conference yesterday, that we protect people's lives and peace whatever happens," Kato added.

- 'Wake-up call' -

China's state-run media launched a broadside against the relaxation of rules, casting it as a threat to Asian security.

"The Japanese government is eager to break through the post-war system," wrote the ruling Communist Party's flagship People's Daily newspaper in an editorial penned under the name "Zhong Sheng", a homophone for "Voice of China".

It called the Abe government's move "a dangerous signal, as well as a wake-up call".

Tokyo and Beijing have long been at odds over islands in the East China Sea, and Beijing has argued that a reinterpretation of Japan's pacifist constitution could open the door to remilitarisation of a country it considers insufficiently penitent for its actions in WWII.

Tokyo has repeatedly refuted the charge it is a wolf in sheep's clothing.

"We shall never repeat the horror of war," Abe said Tuesday. "With this reflection in mind, Japan has gone on for 70 years after the war. It will never happen that Japan again becomes a country which goes to war."

China, home to the world's largest military, far outnumbers rival Japan in manpower, ships, aircraft and defence spending.

China's official defence budget last year came to $119.5 billion, while according to the International Institute for Strategic Studies' Military Balance 2014 report, released in February, Japan's total was $51 billion.

South Korea expressed unease about Japan's change, which it characterised as a "serious alteration" of its pacifist policy, and called on Tokyo to "abandon historical revisionism".

The United States, a potential beneficiary of the move, welcomed the change, which it said was a right "under the UN Charter to collective self-defence".