WASHINGTON (AFP) US President Barack Obama will call for unprecedented global action to secure nuclear stockpiles and keep weapons-grade material out of the hands of extremists, at a two-day summit opening Monday (today). With many world leaders arriving at the weekend, a security blanket was draped over the US capital as Obama headed into separate bilaterals Sunday with the prime ministers of nuclear-armed rivals India and Pakistan. It will be the largest global gathering on US soil in 65 years, and with small armies of security forces deployed in Washington is leaving nothing to chance to protect leaders at the summit. The White House says Obama called the 47-nation conference to put together the most coordinated effort yet to tackle what one adviser called the most dangerous security threat haunting America, and the rest of the world. It is absolutely fundamental to view this summit with the starting point of the grave nature of the threat of nuclear terrorism, said Ben Rhodes, Deputy US National Security Advisor. Obamas Secretary of State Hillary Clinton was also pressing the case for greater vigilance, stressing Sunday that the threat of nuclear terrorism has increased and that other countries were only now recognising the danger. The nuclear security summit - the biggest gathering of world leaders led by a US president since 1945 - marks Obamas boldest effort yet to exercise global leadership on one of his principal foreign policy themes, non-proliferation. Obama wants fellow leaders to agree to his time frame for securing all nuclear materials within four years and a final summit communique will likely issue calls for tougher prosecutions of traffickers in weapons-grade materials. He enters the meeting - an attempt to halt the theft, sale or smuggling of unsecured nuclear materials - with his political standing enhanced by a new disarmament deal with Russia and by enacting historic domestic health care reform. Obama will also flex diplomatic muscle on the sidelines of the summit, with a flurry of one-on-one meetings with other leaders, including Chinese President Hu Jintao. The White House also announced that Obama would meet his Ukrainian counterpart Viktor Yanukovych for the first time on Monday. Some key world leaders will not be at the summit however, including Israeli Prime Minister Benjamin Netanyahu, who has a tense relationship with Obama and reportedly feared Muslim states would raise Israels undeclared nuclear status. British Prime Minister Gordon Brown, fighting for his job ahead of an election next month, also sent his excuses, as did Australian Prime Minister Kevin Rudd who is embroiled in his own health care push. The summit itself will focus primarily on separated plutonium and highly enriched uranium stocks, rather than radiological dirty bombs, which US officials see as a threat, but less potentially catastrophic than nuclear devices. While the summit is designed at keeping nuclear material out of the hands of extremist groups, two states not at the summit, Iran and North Korea, will cast a shadow due to the confrontations over their nuclear programs. Washington is leading an effort to frame a new and biting set of sanctions on Iran, and will seek to lock in Hus cooperation, after months trying to bring China and Russia on board. The meeting is the fruition of a long-term effort by Obama to deprive terrorist groups of the means to build nuclear weapons. US officials are expecting the nations to agree a series of their own security steps and help pay to put the stocks of less well-off countries under lock and key.