For officially pacifist Japan, North Korea's rocket launch over its territory served as a sobering reminder that it is well within firing range of the volatile communist regime.

Less than three years after Pyongyang declared it had tested a nuclear bomb, it was able to fire an intercontinental ballistic missile high over Japan, largely keeping to the trajectory that it had announced before the launch.

Japan has taken the threat seriously.

For the first time since World War II, Tokyo issued advance orders to its armed forces to shoot down a missile or its debris should these veer off the declared course and threaten Japanese territory.

In the first real-life test of its seven-billion-dollar "missile umbrella," Japan deployed three destroyers at sea and five Patriot missile batteries on land, cooperating closely with its main ally, the United States.

North Korea had warned last week that it would see any intercept as an act of war and threatened to "mercilessly deal deadly blows not only at the already deployed intercepting means but at major targets."

Washington has often said the rocket could hit Alaska or Hawaii -- but the threat is far more immediate for Tokyo, located just over 1,000 kilometres (about 650 miles) from North Korea's Musudan-ri launch pad.

"Japan lives in a testy community," said Sheila Smith, a Japan and Asian security expert at the Council on Foreign Relations (CFR) in Washington.

"North Korea's nuclear ambition, its quest for effective long-range delivery capability and its willingness to ignore international sanction make it a volatile neighbour."

In a sign of frayed nerves, Japan issued a false alert Saturday, then closely tracked the actual launch Sunday.

In the end, the rocket cruised high over northern Japan and harmlessly splashed into the Pacific -- all within less than 20 minutes.

Japan held its fire but quickly sent off diplomatic salvos, seeking condemnation of North Korea at the UN Security Council, an effort seen likely to be stalled by veto-wielding China, North Korea's fellow communist ally.

Japanese media reacted furiously to the launch.

The top-selling Yomiuri Shimbun called for tougher sanctions over the "reckless and dangerous provocation that stokes tension in the international community and directly threatens its peace and stability."

Pyongyang's defiant rocket launch was also seen likely to strengthen calls for Japan to add military muscle to its Self-Defence Forces.

The Sankei Shimbun said in an editorial: "We need to promote missile defence further, as urged by a ruling-party taskforce, in order to make interception possible even without an advance notice. The tension is certainly rising."

The conservative newspaper also called for debate on Japan acquiring the military hardware for a preemptive strike, saying that "currently, Japan has no capability to prevent the North firing a missile by attacking its base."

Hideshi Takesada, head of the National Institute for Defence Studies, told AFP the launch meant "a fresh challenge" for Japan's defence posture.

"Japan may have to consider a more practical approach to handling North Korea's missile threat and faces the need for debate over possible possession of more proactive means, such as longer-range cruise missiles," he said.

Such debate is still sensitive in Japan, where the post-war constitution bans any offensive military capabilities.

Japan has sent troops to missions in Iraq, off Afghanistan and to join anti-piracy policing near Somalia -- but its troops have usually served away from the frontlines and have yet to fire a shot in anger.

Japan's Asian neighbours remember its past military aggression well.

North Korea, once colonised by imperial Japan, routinely attacks Tokyo as a neo-colonial aggressor, in the acerbic language it also uses to lambast what it calls "imperialist America and its South Korean flunkies."

The CFR's Smith said that, increasingly, "Japan is having to face the real possibility of being on the frontline of a conflict.

"And with all that is at stake in Northeast Asia, it will need to develop both the military and diplomatic options that will best assure good decisions looking forward."