If al-Qaida acquired nuclear weapons it "would have no compunction at using them," President Barack Obama said Sunday on the eve of a summit aimed at finding ways to secure the world's nuclear stockpile.

"The single biggest threat to U.S. security, both short-term, medium-term and long-term, would be the possibility of a terrorist organization obtaining a nuclear weapon," Obama said. "This is something that could change the security landscape in this country and around the world for years to come."

"If there was ever a detonation in New York City, or London, or Johannesburg, the ramifications economically, politically and from a security perspective would be devastating," the president said.

"We know that organizations like al-Qaida are in the process of trying to secure nuclear weapons or other weapons of mass destruction, and would have no compunction at using them," Obama said.

The Nuclear Security Summit of more than 40 world leaders in Washington this week is aimed at securing "loose nuclear material," Obama said. He was holding one-on-one meetings Sunday with several of those leaders.

He said other world leaders have offered "very specific approaches to how we can solve this profound international problem."

Obama singled out South Africa for giving up its nuclear program, and said it "has been a strong, effective leader in the international community on nonproliferation issues. South Africa has special standing in being a moral leader on this issue."

South African President Jacob Zuma was among the leaders Obama met with Sunday at Blair House, across from the White House. Others included Indian Prime Minister Manmohan Singh and Kazakh President Nursultan Nazarbayev.

"I feel very good at this stage in the degree of commitment and a sense of urgency that I have seen from the world leaders so far on this issue," Obama said. "We think we can make enormous progress on this, and this then becomes part and parcel of the broader focus that we've had over the last several weeks."

Last Thursday, Obama and Russian President Dmitri Medvedev signed a new Strategic Arms Reduction Treaty that reduces each side's deployed nuclear arsenal to 1,550 weapons. Earlier in the week, Obama approved a new nuclear policy for the United States, vowing to reduce America's nuclear arsenal, refrain from nuclear tests and not use nuclear weapons against countries that do not have them.

Obama said securing loose nuclear arms is "a central part of the process, but probably the most urgent one and the one we are most concerned with in the short term."