"That's material that will never fall into the hands of terrorists," Obama said at a news conference at the conclusion of the Nuclear Security Summit in Washington.
Non-proliferation efforts mean the entire continent of South America is free of nuclear materials. If Poland and Indonesia meet commitments this year, Central Europe and Southeast Asia will follow suit, he said.
"As terrorists and criminal gangs and arms merchants look around for deadly ingredients for a nuclear device, vast regions of the world are now off-limits, and that's a remarkable achievement," Obama said, admitting that much work remains.
ISIS, also known as ISIL, and the threat of nuclear terrorism was a major focus of this year's summit, the fourth of its kind.
President Barack Obama said that as ISIS comes under greater international pressure in Syria and Iraq, the group will likely try to conduct more attacks elsewhere.
"As ISIL is squeezed in Syria and Iraq, we can anticipate it lashing out elsewhere, as we've seen most recently and tragically in countries from Turkey to Brussels," Obama said at a special session devoted to the group earlier Friday.
ISIS still controls swaths of Iraq and Syria, however, and in recent weeks claimed attacks in Brussels and Turkey.
Evidence discovered after the Paris attacks of November -- hours of footage tracking the movements of a Belgian nuclear official -- suggest ISIS' interest in weapons of mass destruction.
Obama said the work done by leaders who gathered from around the globe to discuss nuclear security would help reduce the chances of terrorist groups getting their hands on a nuclear weapon.
"No terrorist group has yet succeeded in getting their hands on a nuclear device. Our work here will help ensure that we're doing everything possible to prevent that," Obama said. "This is a threat to us all."
Obama had warned earlier that the world "cannot be complacent" about nuclear safety as the "threat of nuclear terrorism" continues and evolves.
The ongoing challenge, Obama told his counterparts, is to prevent groups like ISIS, which have sought nuclear material, from obtaining it or any weapon of mass destruction.
If terrorists succeed, Obama said, "They would certainly use it to kill as many as possible."
Obama opened Friday morning by focusing on his major achievement on the nuclear front during his time in office, the agreement to curtail Iran's program -- though the nascent deal will be tested in the months and years ahead.
Obama pointed to the agreement as an example of the kind of "substantial success" that could be achieved through concerted diplomacy.
The agreement was not easy to reach, Obama said, but "does remind us that when international community stands as one, we can advance our common security."
Iran is not attending the gathering, which addresses a host of issues related to nuclear security, including better security, better inspections and agreements to remove or reduce stockpiles of highly enriched uranium.
In his remarks, Obama pointed to steps Iran has taken that mean the timeframe it would take for Tehran to build a nuclear weapon has been pushed back from two or three months to a year.
"President Obama is bringing together almost 50 countries here in the nuclear security summit not only to talk about protecting nuclear materials, but that dangerous section between terrorism and nuclear weapons," Brett McGurk, Obama's envoy for the global coalition to counter ISIS, told CNN's Wolf Blitzer on "The Situation Room."
"One of the networks is this chemical weapons network that ISIL has inside Iraq, particularly near Mosul, but we're uprooting that network as we speak," McGurk said, using the U.S. government's name for ISIS.
Concerns over ISIS' nuclear ambitions have grown since Belgian authorities discovered video footage of a Belgian nuclear official in the apartment of a key member of the ISIS-linked terrorist cell that was behind the deadly Paris and Brussels attacks.
The 10 hours of surveillance footage was found in the Brussels residence of Mohammed Bakkali, a suspected planner of the November Paris attacks. Bakkali was arrested on November 26.
"Certainly the video footage is of concern and suggests there is at least some interest by (ISIS) in securing nuclear materials," said Laura Holgate, the White House's senior director for weapons of mass destruction and terrorism.
In a briefing with reporters Tuesday, Holgate said there was no indication the video surveillance of the Belgian nuclear official was part of a "broader plan" by ISIS to acquire nuclear materials.
"We don't have any information that a broader plot exists," she said.
Experts warn the threat of ISIS obtaining radiological materials for a so-called "dirty bomb" is real, however.
They note such materials are easily found in hundreds of commercial and industrial facilities in Europe, and that the mechanics to assemble and detonate such a crude device are simple and well-known.
"ISIS presents a nuclear threat like we've never seen," said Joe Cirincione, president of the Ploughshares Fund, which seeks to prevent the spread of nuclear weapons.
"We think we now have a group that's sophisticated enough, rich enough and has a global network big enough that they can get the material and deliver it with ease," he noted.
Cirincione said that ISIS would only need 10 grams of radiological material and a small amount of conventional explosive to fashion a dirty bomb for a major city capable of rendering 10 square city blocks "uninhabitable for years" with potentially devastating physiological, medical and economic effects.
Noting that ISIS had "penetrated the heart of Europe" and that radiological material was poorly secured throughout Europe, "everyone who has looked at this intensely is surprised it has not happened."
"If the terror threats continue to grow and our controls don't grow faster, it's inevitable that we'll see a dirty bomb used," he warned.
In a new report, "Preventing Nuclear Terrorism: Continuous Improvement or Dangerous Decline?," a group of experts from Harvard University's Belfer Center for Science and International Affairs found that since the last nuclear security summit two years ago, "security for nuclear materials has improved modestly -- but the capabilities of some terrorist groups, particularly (ISIS), have grown dramatically, suggesting that in the net, the risk of nuclear terrorism may have increased."
The report found ISIS' in-depth monitoring of a top scientist at the Belgian facility with substantial stocks of weapons-grade uranium among several "worrying indicators" about ISIS' nuclear ambitions.
The Belfer Center's Matthew Bunn also noted the case of Ilyass Boughalab, a former worker in "the vital area" of a Belgian nuclear power plant who traveled to Syria to join a militant group.
He acknowledged that the Belgians have since taken some action to secure their nuclear facilities, including adding cameras, regulations and armed guards, but he said a "more intense effort to lock down this nuclear material" was needed worldwide.
This is why the report said leaders gathering at this year's Nuclear Security Summit face "an important crossroads," with additional cooperation and funding to enhance nuclear security badly needed.
"We don't know what the terrorist threat is going to look like two years, five years, 10 years from now," Bunn said. "And to me that's even stronger reason to lock down all the ingredients of a potential nuclear recipe, wherever they may be so that whatever that terrorist threat is, they can't get access to that kind of material."