<p><br /> Barack Obama has optimistically opened a 47-nation nuclear summit, boosted by Ukraine's announcement that it will give up its weapons-grade uranium.<br /> </p><p><br /> More sobering was the warning by Obama's counterterrorism chief that al-Qaida is vigorously pursuing material and expertise for a nuclear bomb.<br /> </p><p><br /> Kicking off the two-day summit, the US President welcomed the heads of the national delegations in front of television cameras, before heading to a working dinner behind closed doors.<br /> </p><p><br /> The first to be welcomed was Israeli Deputy Prime Minister Dan Meridor, who attended the summit after his PM Benjamin Netanyahu opted not to attend.<br /> </p><p><br /> Israeli insiders said Netanyahu expected Turkey and Egypt to use the conference as a platform to challenge him over his country's widely assumed nuclear arsenal, which Israel has never acknowledged.<br /> </p><p><br /> While Obama welcomed leaders from around the world, notable exceptions were Iran and North Korea.<br /> </p><p><br /> Presidential aides billed the summit as the largest gathering of world leaders hosted by an American president since the 1945 conference in San Francisco that founded the United Nations.<br /> </p><p><br /> The talks are a centrepiece of Obama's broader agenda for ridding the world of nuclear weapons, a long-term process that he says should include gradual disarmament by the nuclear powers, stronger steps to head off a nuclear arms race and more urgent action to lock down tons of plutonium and highly enriched uranium that are the key building blocks of atomic weapons.<br /> </p>
