BRUSSELS (Reuters) - NATO has joined world powers  diplomatic efforts to stop North Korea s missile program but it cannot yet rely on its U.S.-built shield to defend Europe, experts and diplomats said. The United States says the shield, more than a decade in the planning, is needed to protect against so-called rogue states, a term U.S. officials have used to refer to North Korea and Iran.  But with Berlin, Paris and London potentially within striking distance of North Korea s missiles from next year, officials say the U.S.-led alliance s system needs more radars and special interceptors to destroy a rocket from Pyongyang.  The NATO shield in its current state lacks the reach and early warning radars to shoot down North Korean rockets. It s a weak link,  said Michael Elleman, a missile defense analyst at the International Institute for Strategic Studies (IISS).  Early tracking is also difficult because North Korean missiles would be flying over Russia, where NATO obviously cannot put radars,  he added. The sort of interceptor needed to shoot down North Korean ballistic missiles could breach a Soviet-era arms control agreement between the United States and Russia because of its greater range, arms experts say. Moscow has long objected to U.S. missile shield plans, saying their real aim is to neutralize Russia s own nuclear arsenal, rather than meet the perceived threat from  rogue states . Russia s strategic concerns would, therefore, make it hard to renegotiate the 1987 Intermediate Range Nuclear Forces (INF) treaty, something arms experts say would be required if a North Korean missile shield were to be fully effective.  Alliance planning to confront any threat from Pyongyang is in its infancy. Following North Korea s country s sixth and most powerful nuclear test on Sept. 3, two senior NATO diplomats told Reuters that protection against the North Korean threat was only beginning to be considered at NATO headquarters in Brussels. That was despite a more forceful diplomatic tone on the crisis and warnings on the scale and immediacy of the threat from U.S. President Donald Trump s new ambassador to NATO, France s defense minister and the alliance s deputy head. While analysts do not expect North Korea to have a reliable intercontinental ballistic missile until next year at the earliest, NATO s European allies could become a target as a way of threatening their closest partner, the United States, a third NATO diplomat said, stressing that was only speculation. The United States switched on its $800 million European missile defense umbrella in May last year at a site in Romania to protect against Iranian rockets.  The system, controlled from a NATO base in Germany, includes radars and interceptors stretching from eastern Europe to the Mediterranean.  A final site in Poland should be ready by late 2018, extending the European umbrella from Greenland and the Azores.    To shoot down a ballistic missile from North Korea would require a new generation of interceptor, the Block II, which is still in development. It is capable of downing ballistic rockets earlier and at a much higher altitude. However, Elleman said that U.S. missile sites in Alaska and California, as well as in Japan and South Korea, were likely to be given priority before Europe, when they are ready in 2018.  There will be a lot of competition for the assets,  he said. 