WASHINGTON (Reuters) - U.S. President Barack Obama will seek to build momentum with China in his push for sanctions on Iran as world leaders assemble in Washington on Monday for an unprecedented nuclear security summit. Obama will hold talks with Chinese President Hu Jintao before hosting high-level delegations from nearly 50 countries for the opening of the global conference, where the focus will be on how to prevent nuclear terrorism. In the one-on-one meeting with Hu, Obama hopes to cement Chinas commitment to help ratchet up pressure on Iran over its nuclear program after Beijing agreed to join serious talks about possible new U.N. sanctions on Tehran. The two leaders will also try to nurture a thaw in Sino-U.S. relations after tensions spiked in recent months over a range of issues. Financial markets will be seeking further signs of China giving ground over its currency valuation. The Washington summit is the culmination of a hectic week of nuclear diplomacy for Obama and comes a year after he laid out a vision of a world free of atomic weapons. It follows close on the heels of Obamas unveiling of a revamped U.S. nuclear doctrine limiting the use of atomic arms and the signing of a landmark post-Cold War treaty with Russia pledging to cut their nuclear arsenals by a third. At home, Obamas conservative critics say his arms-control strategy is naive and could compromise U.S. national security. Despite that, the two-day summit the biggest U.S.-hosted assembly of world leaders in six decades will be a test of Obamas ability to rally global action on his nuclear agenda. Speaking on the eve of the conference, Obama said he expected it to yield enormous progress towards the goal of locking down loose nuclear materials worldwide. We know that organizations like al Qaeda are in the process of trying to secure a nuclear weapon, a weapon of mass destruction that they have no compunction at using, Obama told reporters, calling it the biggest threat to national security. A draft final communiquT shows leaders will pledge to work towards safeguarding all vulnerable nuclear material within four years and take steps to crack down on nuclear smuggling. NOT ON AGENDA BUT ON SUMMITEERS MINDS Iran and North Korea are not on the guest list or the summit agenda. But their nuclear standoffs with the West are sure to figure heavily in Obamas talks with Hu and other leaders like German Chancellor Angela Merkel, who will sit down with the U.S. president on Tuesday after the summit is over. With Obama pushing to get new sanctions in place against Iran within weeks, China after months of delay reluctantly agreed to join in crafting a U.N. resolution. But Obama has yet to completely overcome Beijings scepticism. I think time is pressing and a decision on potential sanctions will need to be made soon, Merkel said in Berlin prior to her departure to the United States. Iran dismissed the U.S. summit and said it would not be swayed by any decisions made there. We already know the results of the Washington summit and whatever they decide is not binding for other countries because they were not invited and are not attending, Ali Asghar Soltanieh, Irans ambassador to the International Atomic Energy Agency, told ISNA news agency. America is the real threat for international security, Soltanieh said, echoing comments by Supreme Leader Ayatollah Ali Khamenei who said the United States was a warmonger speaking of peace which had threatened to attack Iran with nuclear weapons. Iran says its nuclear program is purely peaceful but the West suspects it is a covert drive to develop atomic weapons. The list of leaders in attendance will range from heads of state of traditional nuclear powers like Russia and France to nuclear-armed foes like India and neighbouring Pakistan. Pakistans Prime Minister Yusuf Raza Gilani assured Obama in talks on Sunday his government has appropriate safeguard for its nuclear arsenal. Experts say Pakistans stockpile of weapons-grade material poses a high risk because of internal security threats from the Taliban and al Qaeda. Missing will be Israeli Prime Minister Benjamin Netanyahu, who withdrew fearing Muslim leaders would use the summit as a forum to demand Israel give up its assumed nuclear arsenal. Still, nuclear-defiant Iran will be the summits sub-text. In Prague last week, Obama persuaded President Dmitry Medvedev to keep pressure on Iran, but the Russian leader made clear there remain limits to Moscows support for sanctions. Hus decision to attend the summit is seen as part of a two-way effort to get relations back on track after months of bickering over Chinas currency, its Internet censorship, U.S. arms sales to Taiwan and Obamas meeting with the Dalai Lama. Days after Beijing announced Hus participation, Washington said it would delay a decision scheduled for mid-April on whether to declare China a currency manipulator. China, meanwhile, has signalled it may be close to revaluing its yuan currency. In a pivotal congressional election year, the Obama administration has pressured Beijing to scrap its currency peg, saying it hurts U.S. business and jobs.