WASHINGTON — World leaders declared progress in safeguarding nuclear materials sought by terrorists and wayward nations, even as President Barack Obama acknowledged the task was far from finished.

Closing out a nuclear security summit on Friday, Obama warned of a persistent and harrowing threat: terrorists getting their hands on a nuclear bomb. He urged fellow leaders not to be complacent about the risk of catastrophe, saying that such an attack by the Islamic State or a similar group would “change our world.”

“I’m the first to acknowledge the great deal of work that remains,” Obama said, adding that the vision of disarmament he laid out at the start of his presidency may not be realized during his lifetime. “But we’ve begun.”

Despite their calls for further action, the roughly 50 leaders assembled announced that this year’s gathering would be the last of this kind. This year, deep concerns about terrorism were the commanding focus, as leaders grappled with the notion that the next Paris or Brussels could involve an attack with a nuclear weapon or dirty bomb.

Obama said of the terrorists, “There is no doubt that if these madmen ever got their hands on a nuclear bomb or nuclear material they most certainly would use it to kill as many innocent people as possible.”