US President Barack Obama gathers world leaders in The Hague on Monday to seek ways of preventing a terrorist nuclear attack, at a key summit that risks being overshadowed by the explosive Ukraine crisis.

Over 50 leaders are to attend the Nuclear Security Summit (NSS) at the behest of Obama, who has called a simultaneous meeting of the Group of Seven top economies to discuss further sanctions against Russia.

G7 leaders from Britain, Canada, France, Germany, Italy, Japan and the US will likely meet on Monday, diplomats said, leaving Tuesday free to discuss securing the world's stocks of nuclear material to prevent a group like Al-Qaeda acquiring a nuclear or so-called 'dirty' bomb of conventional explosives wrapped in radioactive material.

Obama has made nuclear security and cutting nuclear weapon stocks a centrepiece of his political legacy, saying in 2009 that nuclear terrorism was "the most immediate and extreme threat to global security".

The Nobel Peace Prize winner set out to secure all vulnerable nuclear material worldwide within four years, with nuclear security summits held in Washington in 2010 and Seoul in 2012. Obama will host a final summit in 2016.

One of the summit's aims is to get countries to give up their stocks of highly-enriched uranium and plutonium, which can be used to make an atomic bomb, in exchange for less dangerous low-enriched uranium.

"A nuclear terrorist event would impact the entire world so it's a global problem that requires a global solution, and the NSS has brought this problem the appropriate political attention," Kelsey Davenport, Non-proliferation analyst at the Arms Control Association, told AFP.

- Widespread impact -

The Dutch hosts, who have mounted an unprecedented security operation around the summit, with much of the sleepy Hague suburb of Scheveningen locked down, have said the summit has three aims: reducing the amount of nuclear material, protecting radioactive material and strengthening international cooperation on nuclear security.

Almost every country has radioactive material that could be used in a "dirty" bomb.

"You might not have a nuclear reactor in your back yard, but you probably have a hospital, with different (radiological) sources, for cancer treatment and sterilisation, I don't think people realise how close to home some of this hits," Michelle Cann, Senior Budget and Policy Analyst at the Partnership for Global Security, told AFP.

"The impact would be so widespread, in so many different areas. That's what makes it so important to act preventively," said Cann.

The summit's final statement is expected to point to the future rather than lay down any hard rules for nuclear security.

"Unfortunately, with the summit process that operates by consensus, the document that comes out of it will be the lowest common denominator," said Davenport.

Observers said individuals may nonetheless make so-called "gift basket" promises of specific action that go beyond the measures included in the final statement.

- Ukraine success story -

Ukraine was one of the big success stories at the 2012 Seoul summit, after they returned all their highly enriched uranium to Russia and converted their nuclear reactors to use low-enriched uranium.

Kiev agreed to give up its nuclear weapons in 1994 in exchange for guarantees of its territorial integrity, but analysts denied that Russia's actions in Crimea and Ukraine would be different if they still had the bomb.

"Frankly, this is an area of the world that's very volatile, and the less nuclear material there is around there, the better. I don't think we can say that Ukraine made a mistake," said Cann, from the Partnership for Global Security.

The White House's top arms control official, Elizabeth Sherwood-Randall, insisted last week that despite the worst crisis between Russia and the West since the Cold War, the US and Russia were continuing to work "effectively" together to prepare the nuclear summit.

Chinese President Xi Jinping, on his first European tour, will hold talks with Obama at the nuclear summit, which Russian Foreign Minister Sergei Lavrov is also expected to attend. Ukraine is sending interim premier Arseniy Yatsenyuk, organisers said.

Despite positive progress in Tehran's nuclear negotiations with the West, Iran is not invited -- "that would send the wrong message," a diplomatic source told AFP -- and nor is nuclear-armed North Korea.

"This summit has the potential to be very successful in setting us up for where we need to be in 2016," said Cann.

But "it's still not clear if this is going to be durable and sustainable once the heads of state stop meeting, and that's the most important question to answer," she said.