Washington ? The U.S.-Russian effort that whisked a cache of weapons-grade uranium out of Yugoslavia this week is part of a larger nuclear materials security program given new urgency after the Sept. 11 attacks.

Experts worry that terrorists or hostile nations may get their hands on enough uranium or plutonium to build a nuclear bomb from one of hundreds of research reactors around the world.

The United States is focusing on 24 reactors in 16 countries that, like the site in Yugoslavia, were built and fueled with help from the former Soviet Union, State Department officials said Friday.

“We want to get at all of them. Some of them are more pernicious than others,” said a top State Department official involved in the program, who spoke on condition of anonymity. “We have plans to address every single one of these facilities.”

The reactors are designed to use highly enriched uranium which can also be used to make nuclear bombs to create nuclear isotopes used for medical treatments and other peaceful purposes. Now, given advances in technology and increased worries about terrorism, there’s no need for those reactors to use bomb-grade uranium, U.S. officials say.

The research reactors are a big worry because they would offer a ready source of precisely the material needed to create a nuclear bomb and security at some of them is frighteningly lax.

“In some cases security is provided by a single sleepy watchman and a chain link fence,” said a report released in May by Harvard University’s Project on Managing the Atom.

The U.S. government is constantly tracking reports and rumors that terrorists and hostile nations are looking for enriched uranium and plutonium. Iraq, for example, had ties with former Yugoslav leader Slobodan Milosevic, though there’s no evidence Milosevic ever gave any nuclear material to Iraq.

“We’re aware on a routine basis of certain countries that go shopping for this material,” the State Department official said, adding, “We’re going to stop them any place we can.”

The United States is working with Uzbekistan to get rid of the highly enriched uranium stockpile at a research reactor in the former Soviet republic, which borders Afghanistan. That reactor has been a worry because an Islamist group with ties to al-Qaida and the Taliban has been blamed for terrorist attacks in Uzbekistan.

The Harvard report raises concerns about a research reactor in the former Soviet state of Belarus and a reactor in Ukraine, and the Energy Department has told Congress it hopes to help Romania get rid of bomb-grade uranium.

Officials declined to name other specific sites for fear of giving terrorists a list of places to get nuclear materials.