Barack Obama has optimistically opened a 47-nation nuclear summit, boosted by Ukraine's announcement that it will give up its weapons-grade uranium.

More sobering was the warning by Obama's counterterrorism chief that al-Qaida is vigorously pursuing material and expertise for a nuclear bomb.

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.

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.

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.

While Obama welcomed leaders from around the world, notable exceptions were Iran and North Korea.

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.

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.