WASHINGTON — President Barack Obama said the possibility of terrorists getting a nuclear device is the "single biggest threat" to US security as he opened a series of meetings with world leaders on preventing the spread of atomic material.Obama talked with five leaders Sunday in Washington and another series Monday to kick off a summit among 46 nations and the US where the president will press his nuclear policy agenda."There's a lot of loose nuclear material around the world" and the summit's focus "is getting the international community on a path to which we are locking down" those supplies, Obama said before meeting with South African President Jacob Zuma. "We know that organizations like al-Qaida are in the process of trying to secure a nuclear weapon."Before meeting with Zuma, Obama met with Prime Minister Manmohan Singh of India and President Nursultan Nazarbayev of Kazakhstan. Talks also are scheduled with Prime Minister Yousuf Raza Gilani of Pakistan and Nigeria's acting president, Goodluck Jonathan. Obama is scheduled to wrap up Monday's bilateral conferences with Chinese President Hu Jintao before the formal opening of the two-day international summit on securing nuclear materials.The Washington summit highlights one of Obama's top foreign policy priorities: curbing the spread of nuclear material that can be used for weapons. The US president wants the nations represented at the summit to reach agreement on a four-year plan to secure separated plutonium and highly enriched uranium that could be smuggled or sold on the black market to al-Qaida and other terrorist groups.The goal is to make sure countries are "not just talking about general statements of support but rather with very specific approaches to how we can solve this profound international problem," Obama said.Neighbors and rivals India and Pakistan both have nuclear arms and the Obama administration has been working to increase cooperation with and between the two countries as they confront extremism in South Asia.A five-year peace process between the countries was halted following the November 2008 assault on Mumbai that left 166 people dead. India blamed terrorist "elements" from Pakistan for the attack and demanded the perpetrators be brought to justice.Last February, Pakistan acknowledged that its territory was used to plot an attack in Mumbai and later filed charges against more than two dozen people for their roles.Obama also is set to meet with the president of Kazakhstan, which gained independence from the former Soviet Union in 1991, inheriting the world's fourth largest nuclear missile arsenal and a nuclear test site where some 460 nuclear weapons tests were conducted during the Cold War.President Nursultan Nazarbayev moved in 1991 to dismantle the test facility and eliminate 104 intercontinental ballistic missiles and 1,410 nuclear warheads.The government of Kazakhstan, which has about one-fifth of the world'snatural uranium supply, has said it wants to develop cutting-edgepeaceful nuclear technologies and supports initiatives for improvingthe safety of nuclear fuel.Nazarbayev has come under criticism for human rights abuses. Accordingto the US State Department's 2009 Human Rights Report those abusesinclude torture of prisoners; arbitrary arrest; no independentjudiciary; and restrictions on freedom of speech.US National Security Adviser James Jones told reporters last weekthat Nazarbayev has "done something very courageous and exemplary forhis country and for proliferation in general."Jones said Obama will "never hesitate to speak up on democracy andhuman rights," and "other subjects" aside from nuclear security willcome up during today's bilateral meetings.