U.S. President Barack Obama on Sunday expressed optimism that countries attending the nuclear security summit in Washington will "make enormous progress" on both the commitment and specific steps to secure nuclear weapons.

"I feel very good at this stage in the degree of commitment and sense of urgency that I've seen from the world leaders" on securing nuclear material, Obama said on the eve of the summit that will see 47 countries come together to confront the possibility of a terrorist organization obtaining a nuclear weapon, which he called "the single biggest threat to U.S. security, both short term, medium term and long term."

"We think we can make enormous progress on this," Obama told reporters before he met with South African President Jacob Zuma at the fringe of the summit.

Obama said organizations such as al-Qaeda terror network are in the process of trying to secure a nuclear weapon, which they "have no compunction at using."

He bemoaned the "situation in which there is a lot of loose nuclear material around the world," and he said he hoped the international community can get behind the goal to lock down the loose material "in a very specific time frame with a specific work plan."