Japan has deployed a mobile defensive missile system near Tokyo for the first time to try to protect the capital in the event of a missile attack.

The Patriot missiles have come into service a year earlier than planned, after North Korea's firing of missiles and a nuclear test last year.

Japan sees North Korea as a real threat to security. This deployment is the first stage in a wider defence plan.

About 50 protesters greeted the arrival of the convoy carrying the missiles.

US co-operation

The Patriot system is capable of shooting down incoming projectiles as they near their targets.

Each launcher can fire up to 16 missiles and has a range of several dozen kilometres.

Should there be fears that a missile attack is imminent, the interceptors could be moved to Air Self Defence Force Bases in the centre of Tokyo to protect government buildings and other potential targets.

For the system to work the Japanese and the United States have to share more data than they have before.

This worries some who question whether further integration of Japanese self-defence forces with the US military is appropriate for a country that is officially pacifist.

The government though is in no doubt that this is a necessary defensive step.

Within three years Japan plans to have 30 launchers at 10 sites around the country, and to equip four destroyers with longer-range missiles in order to build an effective missile shield at sea.