TOKYO - Japan approved on Tuesday plans for a missile early warning system and some ruling party lawmakers suggested Japan should inspect North Korean ships, as a report said the North was preparing to fire a mid-range missile.

Pyongyang's launch over Japan in April of what it said was a satellite-bearing rocket, followed by a nuclear test and several short-range missile launches last week, has raised tensions in Tokyo, which sees itself as a potential missile target.

A space panel headed by Prime Minister Taro Aso agreed to the satellite missile detection plan as part of a new space policy document, a year after Japan dropped a decades-old ban on military use of space.

Japan put four spy satellites into space following North Korea's 1998 launch of a long-range missile that flew over Japan, but the satellite information-gathering system is in need of updating.

The new space policy says efforts would be made to increase the frequency at which photographs are taken of areas of concern, improve image quality and speed up the provision of information. The research will include efforts to develop an early warning sensor, the government said in the document.

Japan, which has a history of rocky ties with North Korea for reasons ranging from its 1910 colonization of the Korean Peninsula to Pyongyang's abduction of Japanese citizens decades ago, has built up a missile defense system in cooperation with the United States.

South Korea's Yonhap news agency said on Tuesday the North was preparing to fire a mid-range missile, capable of hitting almost anywhere in Japan, after reports a long-range missile could be fired in weeks.

In a bid to put pressure on North Korea, a group of conservative ruling Liberal Democratic Party (LDP) lawmakers said they had begun work on a bill that would allow Japan to inspect North Korean cargo ships, despite restrictions related to the country's pacifist constitution.

Japan is pushing to include compulsory inspection of North Korean cargo ships among sanctions in a new U.N. Security Council resolution, but under current law, could not itself inspect ships unless Japan or nearby areas were under threat.

Japan is a member of the Proliferation Security Initiative (PSI), launched by the United States in 2003 to inspect ships and planes in an effort to curb the spread of nuclear technology and other weapons. But Japan's military activities are limited by its postwar pacifist constitution.

Ichita Yamamoto, who heads the group, said it would try to pass a bill allowing Japan to inspect ships in the current parliamentary session ending in late July.

The fact that the LDP is discussing this and that we are coming up with draft bills could send a message to North Korea, he told reporters after the group's meeting.

But it was unclear how much support the bill would gather, because inspecting ships in international waters may be seen by North Korea as a step close to a declaration of war.

(Additional reporting by Yoko Nishikawa; Editing by Jerry Norton)

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