A nuclear watchdog group warns that, despite efforts to secure nuclear material, nations are still vulnerable to theft, as well as acts of sabotage or cyberattacks, which could enable terrorists to obtain nuclear materials or trigger a dangerous meltdown.

Twenty-four states "still have one kilogram (2.2 pounds) or more of weapons-usable nuclear materials…[and] nearly 2,000 metric tons (4.4 million pounds) of weapons-usable nuclear material remain stored around the world, much of it still too vulnerable to theft," said Sam Nunn, chair of the Nuclear Threat Initiative, in the introduction to a new report titled "2016 NTI Nuclear Security Index: Theft and Sabotage."

The NTI Index ranks nations in terms of their safeguards for keeping nuclear explosive materials protected.

Alarmingly, the report found that more than 80 percent of all nuclear explosive materials are held by militaries "whose practices and safeguards are not covered by international agreements on security of such materials." The report urged all nuclear weapons states to agree on a set of security precautions to implement.

"We are in a race between cooperation and catastrophe, and the world's leaders must run faster," said Nunn, a former Senate Armed Services Committee Chair, in commenting on the reports' findings, according to The Center for Public Integrity.

In 2014, the index listed seven countries that were taking the most important steps in preventing theft of nuclear materials through their elimination, but two years on only one country, Uzbekistan, had done so.