World leaders called on Tuesday for countries to minimise their stocks of highly enriched nuclear fuel to help prevent Al Qaeda-style militants from obtaining atomic bombs, at the end of a two-day summit overshadowed by the crisis in Ukraine.

Holding a third nuclear security summit since 2010, leaders from 53 countries - including US President Barack Obama - said much headway had been made in the past four years.

But they also made clear that many challenges remained and stressed the need for increased international cooperation to make sure highly enriched uranium (HEU), plutonium and other radioactive substances did not fall into the wrong hands.

The US and Russia set aside their differences over Crimea to back the meeting's final statement aimed at enhancing nuclear security around the world, together with other big powers including China, France, Germany and Britain.

"We encourage states to minimise their stocks of HEU and to keep their stockpile of separated plutonium to the minimum level, both as consistent with national requirements," said the communique, which went further in this respect than the previous summit, in Seoul in 2012.

A fourth summit will be held in Chicago in 2016. The summit process began in Washington in 2010.

Analysts say that radical groups could theoretically build a crude but deadly nuclear bomb if they had the money, technical knowledge and fissile materials needed.

Obtaining weapons-grade nuclear material - HEU or plutonium - poses the biggest challenge for militant groups, so it must be kept secure both at civilian and military sites, they say.

Referring to a push to use low-enriched uranium (LEU) as fuel in research and other reactor types instead of more proliferation-prone HEU, the summit statement said: "We encourage states to continue to minimise the use of HEU through the conversion of reactor fuel from HEU to LEU, where technically and economically feasible.

"Similarly, we will continue to encourage and support efforts to use non-HEU technologies for the production of radio-isotopes, including financial incentives," it said. - Reuters