Japan has launched a spy satellite aimed at monitoring North Korea.

The rocket, carrying a radar satellite, took off from the southern island of Tanegashima after three delays due to bad weather.

"The satellite has just gone into orbit and is operating normally," said a spokesman at the Cabinet Satellite Intelligence Centre, which operates the information-gathering system.

It completes a set of four satellites to monitor potential threats from North Korea, which fired a missile over Japan in 1998.

With the full complement of four satellites, Japan will be able to monitor any point on Earth once a day.

The rocket was also carrying an experimental optical satellite, aimed at improving the level of detail obtained from the next generation of satellites.

At present, Japan's spy satellites can distinguish objects 1 metre or more in diameter, whereas US military satellites are said to be able to pick up items one-tenth the size.

Japan has encountered difficulties with other space projects.

Its biggest-ever satellite, launched last year and designed to improve mobile phone communications, is itself having communication difficulties.

- Reuters