In a secret operation to secure nuclear material, the United States has helped Ukraine return to Russia enough uranium to build two atomic bombs.
This week's removal of more than 110lb (50kg) of highly enriched uranium followed a pledge by Ukrainian President Viktor Yanukovych to get rid of all of his country's highly enriched uranium by April 2012.
The material will be blended down in Russia, rendering it useless for bomb making.
Details of the operation were provided by the National Nuclear Safety Administration.
Mr Yanukovych agreed to give up the uranium in a deal announced at a nuclear security summit hosted by President Barack Obama in April.
As an incentive, the United States is providing replacement low-enriched uranium that can be used for Ukraine's research reactors.
The summit deal also has the United States building a 25 million US dollar "neutron source facility" nuclear research project for Ukraine, the administration said.
The facility will be able to produce 50 different types of medical isotopes, using only low-enriched uranium.
The US nuclear administration's chief, Thomas D'Agostino, called the uranium removal operation an important step toward Mr Obama's goal of securing the world's nuclear material within four years.
He praised Ukraine for helping ensure its bomb-making material would not fall "into the wrong hands".