World leaders should do more to safeguard their nuclear facilities to prevent "madmen" from extremist groups like Islamic State from getting their hands on a nuclear weapon or radioactive "dirty bomb", US President Barack Obama said.

Speaking at a nuclear security summit in Washington, Obama said no group had succeeded in obtaining bomb materials.

But he said al Qaeda had long sought them and cited actions by Islamic State militants behind recent attacks in Paris and Brussels that raised similar concerns.

Militants were found to have videotaped the daily routine of a senior manager of a Belgian nuclear plant, Obama said.

At a press conference at the conclusion of the nuclear security summit, he said he invited participating nations to work on improving intelligence sharing to prevent terrorist attacks.

Obama hosted more than 50 world leaders, including British PM David Cameron, at his fourth and final summit.

During the 'family photograph' of the leaders at the summit on Friday, Obama was snapped apparently showing a 'peace sign'. But he was actually telling the photographer that they had to wait for two late-running delegates.

The summit focused on efforts to lock down vulnerable atomic materials to prevent nuclear terrorism, which he called "one of the greatest threats to global security" in the 21st century. North Korea's nuclear defiance was also high on the agenda.

A boycott by Russian President Vladimir Putin, unwilling to join in a US-dominated gathering at a time of increased tensions between Washington and Moscow over Ukraine and Syria, added to doubts that the meeting would yield any major decisions.

U.S. President Barack Obama said on Friday that Russian President Vladimir Putin's focus on building up the military at the expense of economic development has slowed progress on a next phase of nuclear arms reductions.