"Terrorists are actively seeking nuclear weapons and materials to make them," said the report by Harvard University's Managing the Atom project.
The group annually assesses progress, and the lack of it, being made to protect nuclear weapons and other nuclear material that, if stolen by terrorists, could lead to a nuclear explosion.
Matthew Bunn, the report's author, said Wednesday that "we are making real progress" in increasing security for nuclear material and warheads in Russia but that there remain "gaping holes" in the protection of nuclear materials in many areas of the world.
"The gap has narrowed, but it remains a dangerous gap that needs to be closed," Bunn said in a conference call with reporters.
Charles Curtis, president of the Nuclear Threat Initiative, a private advocacy group that commissioned the report, said: "This is not a question of the glass half full or half empty." He noted that while there has been a measure of progress it takes only a few pounds of plutonium or highly enriched uranium to fashion a bomb.
"We are not yet treating it as the No. 1 security threat to our nation and the world," said Curtis, who was deputy energy secretary in the Clinton administration.
The report said the essential ingredients for a nuclear weapon exist in more than 40 countries "and there are scores of sites that are not secure enough to defeat the capabilities that terrorists and criminals have demonstrated" should they seek to steal such material.
Neither the U.S. government nor the International Atomic Energy Agency has "a prioritized list assessing which facilities around the world pose the most serious risk of nuclear theft," the report said.
It said from information that is available the highest risks of nuclear theft are in Russia, Pakistan and more than 140 research reactors, often minimally guarded, in various countries that still contain highly enriched uranium suitable for making a nuclear bomb.
"Pakistan's nuclear stockpiles are comparatively small, and are believed to be heavily guarded. But face huge threats from armed jihadi groups and nuclear insiders with a demonstrated willingness to sell sensitive nuclear technology," said the report.
Other nuclear theft risks include the large-scale transport of civilian plutonium in developing countries, including China and India.
"We need an urgent campaign to lock down all of these stockpiles ... everywhere they exist in the world," said . He noted that there is no agreement between the U.S. and China on securing that country's weapons-related nuclear material.
William Tobey, in charge of nuclear nonproliferation at National Nuclear Security Administration, said there is "a great sense of urgency" about improving security at the former Soviet locations and reducing the risks at research reactors.
He said the agency has accelerated the program to remove highly enriched uranium from research reactors, plans to complete security upgrades at Russian nuclear sites by the end of 2008, and will have radiation detection devices at all Russian border crossings by the end of 2011, six years ahead of schedule.
The report acknowledges progress that has been made in U.S. efforts to increase security over nuclear weapons materials in the former Soviet Union. It said that 70 percent of the buildings with such material have had security improvements.
While the U.S. global threat reduction initiative has expanded efforts to remove highly enriched uranium from 129 research reactors across the globe only 48 have been converted to low-enriched uranium or shut down as of the end of 2006.
"Roughly half of the research reactors operating with around the world today are still not covered by the conversion effort," the report said.