US President Barack Obama said Sunday that all highly enriched uranium had been removed from Ukraine, hailing it as an important step towards curbing the threat of nuclear terrorism.

Obama made the announcement on the eve of a nuclear security summit in Seoul gathering leaders or top officials from 53 nations to look at ways of locking up fissile material that could be used to build thousands of terrorist bombs.

"Just today we saw another important step forward. We learned that the Ukraine has completed the removal of highly enriched uranium from its territory," Obama told a news conference in the South Korean capital.

"I believe it is a preview of the kind of progress we are going to see over the next two days in confronting one of the most urgent challenges to global security -- the security of the world's nuclear weapons and preventing nuclear terrorism."

This week's gathering in Seoul is a follow-up to an inaugural summit that Obama hosted in Washington two years ago.

Ukraine pledged at that summit to remove its entire stock of highly enriched uranium, which can be used to make nuclear bombs, and the process of moving it to Russia for secure storage is now complete.

To replace the stock in civilian atomic energy, Ukraine was sent a small amount of low enriched uranium unsuitable for the manufacture of nuclear weapons, according to the government in Kiev.