The possibility that terrorists could steal nuclear materials remains a “real and urgent danger” despite the significant progress that has been made to restrict access to those materials, according to a report released Wednesday by the “Managing the Atom” project at the Kennedy School of Government.

The report says that the essential ingredients needed to make nuclear weapons exist in over 40 countries and that terrorists are actively pursuing these dangerous materials.

“The threat of nuclear terrorism is a continuing one,” said Matthew Bunn, author of the report and the project’s senior research associate.

According to the report, some progress has been made in pursuing programs to guard and reduce the vulnerable stockpiles of nuclear materials, particularly in Russia where a cooperative threat reduction program with the United States has improved the security of those stockpiles. For security reasons, the report does not say which locations have particularly poor security.

Yet despite the recent improvements, the report says that steps to minimize the nuclear threat are insufficient, citing the danger posed by recent breaches including terrorist teams carrying out reconnaissance at nuclear warhead storage sites in Russia.

“A gap continues to exist between urgency of threat and pace of our response,” Bunn said.

Bunn devotes a significant portion of the report to stressing the urgent need to combat nuclear proliferation, warning of the repercussions of a “terrorist mushroom cloud over the cinders of a major city.”

He details an action plan for reducing the risk of nuclear terrorism that includes global partnership commitments, comprehensive lists of stockpiles all over the world, and the appointment of a senior official in the U.S. government whose specific purpose would be to keep nuclear weapons out of terrorist hands.

“Nuclear terrorism is a tremendously important problem that needs urgent attention and this report is one of the best sources that any policymaker or interested citizen can consult to learn about the problem,” said Martin Malin, the executive director for “Managing the Atom.”