The White House has welcomed a decision by the former Soviet republic of Belarus to eliminate its stocks of highly-enriched uranium by 2012.
White House spokesman Robert Gibbs called the move "a significant step forward."
US Secretary of State Hillary Clinton earlier won the pledge from Belarus Foreign Minister Sergei Martynov after talks on the sidelines of the Organisation for Security and Cooperation in Europe (OSCE) summit in the Kazakh capital Astana.
"This removal represents a significant, as I said, step for President (Barack) Obama's worldwide effort to secure nuclear material," Gibbs said on Wednesday.
The United States says it intends to extend technical and financial assistance to support this effort. Gibbs noted that South Korea has agreed to invite Belarus to a 2012 nuclear security summit contingent upon the country completing its removal of highly enriched uranium.
In highly-enriched form, uranium can be used to form the warhead of a nuclear bomb and there have been fears over the security of the stocks held by ex-Soviet republics such as Belarus.
Clinton was quoted as praising the decision "as a sign of progress in efforts to advance nuclear security and non-proliferation."
Belarus' strongman President Alexander Lukashenko had been quoted as saying earlier this year that the country had hundreds of kilograms of highly-enriched uranium and had no intention of eliminating it.
Wednesday's announcement marked a rare breakthrough in relations between Belarus and the United States, which only a few years ago slammed Lukashenko as Europe's last dictator.
However, the unpredictable Belarussian leader has also in recent months made more positive gestures to the West, distancing himself from his traditional allies in the Kremlin.