102 countries have agreed to make their nuclear sites more secure.
US President Barack Obama has urged nations to do more to prevent "madmen" from the Islamic State group obtaining the material to make a radioactive dirty bomb.
He told a nuclear security summit in Washington DC there is a persistent threat of terrorists conducting a nuclear attack.
More than 50 world leaders have gathered in the US capital over the past two days to discuss how to keep extremists from gathering radioactive materials, and other nuclear threats.
Mr Obama said the required 102 countries had now ratified an amendment to an atomic security treaty that would tighten protections against nuclear theft and smuggling.
"Working together, our nations have made it harder for terrorists to get their hands on nuclear material," he told the Nuclear Security Summit.
US officials said they had no explicit evidence that IS had tried to obtain such materials, but added they have no doubt the group is interested in doing so.
Mr Obama mentioned evidence that extremists linked to the Paris and Brussels attacks had filmed a senior manager at a Belgian nuclear facility.
"ISIL has already used chemical weapons - there is no doubt if these madmen ever got their hands on a bomb or nuclear material they would use it to kill as many as possible," he told world leaders.
He warned that a bomb containing fissile material the size of an apple could be catastrophic.
"The smallest amount of plutonium could kill and injure hundreds of thousands of people," Mr Obama said.
"It would be a humanitarian, political, economic and environmental catastrophe with global ramifications for decades.
"It would change our world."
With Russian President Vladimir Putin boycotting the summit, a major deal on reducing global stockpiles of atomic weapons is out of reach.
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.
"My preference would be to bring down further our nuclear arsenal," Obama told reporters at the conclusion of a two-day summit on nuclear security, attended by dozens of world leaders.
Putin boycotted the summit at a time of increased tensions between Moscow and Washington.
Mr Obama told reporters at the conclusion of the two-day summit Russian President Vladimir Putin's focus on building up the military at the expense of economic development.
He said he has not seen the progress he would have liked from Mr Putin "because of the vision that he's been pursuing of emphasising military might over development," and economic diversification.