Sweden recently shipped three kilos (6.6 pounds) of plutonium from its past nuclear research and development activities to the United States for disposal, Foreign Minister Carl Bildt announced on Tuesday.

"This highly sensitive material has now, under high security ... been transferred to the United States for disposal within the framework of the US Global Threat Reduction Initiative (GTRI)," Bildt wrote in an article published in Swedish daily Dagens Nyheter.

The GTRI is aimed at protecting sensitive material to prevent it being used for the production of nuclear devices or in acts of terrorism.

The Swedish consignment was shipped by boat "under great discretion," Bildt said.

"Several countries have previously shipped enriched uranium, but the Swedish contribution breaks new ground because it is the first time that reprocessed plutonium has been transferred within the framework of the GTRI," he added.

"We have received assurances from the United States that the material will not be used for any military purposes, which was naturally one of the conditions we insisted upon."

"Together we need to do our utmost to reduce the amount of nuclear weapons in the world, to prevent them from spreading further, and so that they will never be used," Bildt said.

The Swedish plutonium is from its nuclear research and development program dating back to the 1950s and 1960s.

Most of it comes from the now-closed Aagesta nuclear reactor outside Stockholm, while smaller amounts were acquired "at an early stage" from the United States and Britain for research into nuclear weapons.