The number of countries possessing the makings of a nuclear bomb has dropped by almost one-quarter over the past two years, but there remain “dangerous weak links” in nuclear materials security, says a report.

The study by the Nuclear Threat Initiative said Mexico, Sweden, Ukraine, Vietnam, Austria, the Czech Republic and Hungary have removed all or most of the weapons-usable nuclear materials on their territories since 2012.

That has reduced the number of countries with one kilogram or more of weapons-usable nuclear materials, such as highly enriched uranium, to 25 from 32 two years ago, the study said. 

Among the 25 countries with weapons-usable nuclear materials, the study ranked Australia as having the best nuclear security arrangements, followed by Canada, Switzerland, Germany and Norway. The U.S. was ranked No. 11. The weakest nuclear security is in Israel, Pakistan, India, Iran and North Korea, according to the study, which assessed factors such as accounting methods, physical security and transportation security.

The report said a significant portion of these materials is poorly secured and vulnerable to theft or sale on the black market. Relatively small amounts of highly enriched uranium or plutonium are required to build a nuclear bomb, which is a declared ambition of terrorist groups, said the Washington Post.

“The result of a nuclear blast at the hands of terrorists or a rogue state would be catastrophic -- with dire consequences that would stretch across the globe for economies, commerce, militaries, public health, the environment, civil liberties and the stability of governments,” the report said.