President Barack Obama Sunday warned that Al-Qaeda would not hesitate to use a nuclear weapon against the United States, before hosting a global summit aimed at thwarting such a nightmare scenario.

Obama will seek support from fellow leaders for his effort to secure all loose nuclear material around the world within four years, when he on Monday opens the largest summit chaired by a US president in 65 years.

He conjured up the horrific possibility of a nuclear detonation in New York City, London or Johannesburg, and the serious global economic, political and security trauma that would result, to characterize the gravity of the threat.

"The single biggest threat to US security, both short-term, medium-term and long-term, would be the possibility of a terrorist organization obtaining a nuclear weapon," Obama said on the eve of the two-day summit.

"This is something that could change the security landscape of this country and around the world for years to come."

"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."

Despite the focus on extremist groups, two states, however, Iran and North Korea, which already has the bomb, will cast a shadow over the two-day summit which opens on Monday.

Washington is leading an effort to toughen sanctions within weeks on Iran over its nuclear program, which the United States and allies say is aimed at producing weapons, a charge Tehran denies.

The White House will seek concrete commitments from world leaders on securing stockpiles of separated plutonium and uranium, to ensure that they cannot be stolen, smuggled or sold to extremists.

"The threat of nuclear war... has diminished. The threat of nuclear terrorism has increased," US Secretary of State Hillary Clinton told ABC News.

To kick off his counter-proliferation drive, Obama met Kazakhstan's President Nursultan Nazerbayev and South African President Jacob Zuma -- praising both for handing over or disarming Cold War-era nuclear weapons.

He also held talks with prime ministers Manmohan Singh and Yousuf Raza Gilani of nuclear-armed rivals India and Pakistan.

Obama, who last week signed a landmark disarmament treaty with Russia and laid out a new US nuclear strategy limiting how Washington could use atomic weapons, said he was confident that the summit would garner important progress.

"I feel very good at this stage in the degree of commitment and sense of urgency that I've seen from the world leaders so far on this issue," Obama said.

"We think we can make enormous progress on this."

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 less catastrophic threat than nuclear devices.

US officials hope nations at the summit will agree a series of their own security steps for their own nuclear material, and help pay to put the stocks of less well-off countries under lock and key.

They also expect some leaders to unveil specific actions, similar to Chile's decision to ship a stock of highly enriched uranium to the United States.

The conference is also a precursor to the United Nations Non-Proliferation Treaty review conference next month, seen as another important moment in heading off a future nuclear arms race.

As well as presiding over the summit, Obama will Monday meet Chinese President Hu Jintao in talks likely to focus partly on US hopes that China will let its yuan currency find a market level.

The White House also announced that the US leader would have a bilateral meeting with Turkish Prime Minister Recep Tayyip Erdogan.

The talks will occur just over a week after Turkey decided to return its ambassador to Washington after a row over moves in Congress to brand the World War I massacres of Armenians as genocide.

Turkey is also seeking to revive stalled reconciliation efforts with Armenia. Obama may play a part in that effort, when he meets Armenian President Serzh Sarkisian in a separate bilateral meeting on Monday.

But some key world leaders will not be at the summit, including Israeli Prime Minister Benjamin Netanyahu, who has a tense relationship with Obama and reportedly feared Muslim states would raise Israel's undeclared nuclear status.

The British and Australian prime ministers also sent their excuses.