The United States, Russia and Kazakhstan Tuesday hailed the success of a project to clean up a huge Soviet-era nuclear test site that once contained enough material for a dozen nuclear bombs.

The nations unveiled new details of the operation to secure the former Semipalatinsk nuclear site in remote Kazakhstan at a conference in Seoul devoted to depriving terrorists of the components for an atomic device.

"A significant volume of work has been accomplished by now," said US President Barack Obama, Russian President Dmitry Medvedev and Kazakh president Nursultan Nazarbayev in a joint statement.

"As a result of application of modern physical and technical means the level of security at the former site has been substantially enhanced.

"This work is nearly complete and we consider it a highly successful example of the trilateral cooperation representing our shared commitment to nuclear security and non-proliferation."

In 2000 the Pentagon completed a project to seal 181 test tunnels and 13 shafts at the site, which was used for hundreds of Soviet nuclear tests between 1949 and 1989.

But after the September 11 attacks in 2001 awoke the world to the threat of mass-casualty terrorism, and amid reports people were scavenging the site, US scientists decided more clean-up work was needed.

The three nations launched a joint plan to entomb radiological material in special cement and to plug, collapse and seal tunnels. Security was upgraded around the site, including with a US-provided drone aircraft, to deter and detect intruders.

The White House said Obama's leadership allowed the project to be completed two years ahead of schedule within budget at an estimated cost of $150 million over eight years.

Washington also Tuesday announced another project with France, Belgium and South Korea to convert reactors that run on highly-enriched uranium, which is suitable for nuclear weapons, to use low-enriched uranium.

The White House said that a project to remove all weapons-usable nuclear material from Sweden was also complete.