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 (50 kilograms) 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 $25m (16m) "neutron source facility" nuclear research project for Ukraine, the administration said.
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".
Ukraine gave a major boost to arms control in 1994 when it agreed to surrender the nuclear weapons it inherited after the Soviet Union's collapse.
The removal operation involved 21 specially designed casks for the uranium to be flown on five flights from three cities.
The operation was delayed for days by ice storms in Ukraine. The US also helped deliver some of the replacement fuel to Ukraine.
"This may have been the most complicated operation NNSA has done in recent years," said Andrew Bieniawski, the US agency's associate deputy administrator for global threat reduction.
The uranium came from three research facilities, in Kiev, Sevastopol and Kharkiv.
An additional amount of uranium remains in Ukraine, but the US said the material was on track to be removed by the April 2012 deadline.