WASHINGTON - 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 that could be exploited by terrorist groups with potentially catastrophic results, according to a study released Wednesday.

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. The Nuclear Threat Initiative is a private, non-partisan group that advocates reducing the risk of the spread of nuclear weapons.

"That's a big deal," said Page Stoutland, vice president of the group's nuclear materials security program. "Getting rid of the materials is one less country where somebody could potentially steal weapons-usable material."

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 drop in the number of countries possessing such materials could be seen as modestly encouraging for President Barack Obama's declared ambition to lock down all of the world's highly enriched uranium and plutonium -- the building blocks of a nuclear weapon. There are an estimated 1,400 tons of highly enriched uranium and almost 500 tons of plutonium stored in hundreds of sites around the world.

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 such as al-Qaida.

"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.

The Obama goal, first proclaimed in 2009, will be the focus of a summit meeting of world leaders in the Netherlands in March. And although concern about the security of nuclear materials is generally directed at Iran, North Korea and Pakistan, the study released Wednesday said the United States has its own vulnerabilities.

It cited two recent incidents in the U.S. that point up imperfections in U.S. control of nuclear weapons materials, including a July 2012 break-in by anti-war protesters at the Y-12 complex in Oak Ridge, Tenn., that stores the nation's supply of weapons-grade uranium, makes nuclear warhead parts and provides nuclear fuel for the Navy.

The study by the Nuclear Threat Initiative also cited the firing in October of the No. 2 commander of U.S. Strategic Command, which is in charge of nuclear war planning and would operate nuclear weapons if a president ordered their use. Vice Adm. Tim Giardina was fired amid allegations of involvement with counterfeit gambling chips -- an allegation that raises questions about the potential corruption of nuclear secrets. Giardina has been under investigation by the Naval Criminal Investigative Service; results have not been released.

The Giardina matter, combined with the break-in at Oak Ridge, suggests that "it is dangerous and inappropriate" to take the security of U.S. nuclear materials for granted, the Nuclear Threat Initiative study said.