The environmentalist organisation calculates that there are 520 nuclear weapons deployed in Germany, Italy, Belgium, the Netherlands, Greece and Turkey - placed there in case they are needed to back up Nato's mutual security guarantee.
As the conference on extending the treaty opens in New York, Greenpeace has called for the weapons to be removed. "We want countries at the NPT review to take this issue up," said Hans Kristensen, author of the report.
Most of the weapons are held under the custody of US forces and would be released in time of war. Though this does not break the letter of the NPT, "it contravenes the spirit and the fundamental objectives of the treaty," said Mr Kristensen. "How would we feel if China put nuclear weapons in North Korea on similar terms?"
The US has about 480 nuclear bombs at 15 European bases. They are different variants of the B-61 tactical nuclear bomb, with yields of between one and 175 kilotons. A legacy of the Cold War, they remain in place as political symbols of the links between the US and Europe and as part of the Nato deterrent. Despite the change in relations with Russia, Nato maintains nuclear deterrence at the heart of its policy.
Greenpeace says Britain deploys four squadrons of Tornado aircraft with two to three dozen nuclear WE-177A and B bombs at RAF Bruggen in Germany, with yields of 200 kilotons for the A version and 400 kilotons for the B version. Britain said on 4 April that it would withdraw its nuclear free-fall bombs completely from service by the end of 1998.
Nato's nuclear doctrine and force posture is likely to come under further pressure from developments in European security. The enlargement of the alliance to the east has already posed new problems for its stance on nuclear weapons.