Washington ? President Bush proposed several steps Wednesday to try to prevent the spread of “the world’s most dangerous weapons,” and he detailed the successful breakup of a black-market network that traded nuclear technology out of Pakistan.

“We will proceed as if the lives of our citizens depend on our vigilance — because they do,” Bush said during a speech at the National Defense University in the capital. “Terrorists and terror states are in a race for weapons of mass murder, a race they must lose.”

He said a surprise attack involving a nuclear, chemical or biological weapon represents the “greatest threat before humanity today.”

Among his proposals, almost all of which were focused on the nuclear threat, was a call for tightening of the world’s nuclear Non-Proliferation Treaty and strengthening of the International Atomic Energy Agency, which is the nuclear watchdog for the United Nations.

He also called for an expansion of intelligence efforts aimed at crushing underground weapons networks, such as that headed by A.Q. Khan, the father of Pakistan’s nuclear bomb. Khan last week confessed to selling nuclear technology to Iran, Libya and North Korea.

For the first time Bush publicly detailed Khan’s efforts, along with the U.S. intelligence community’s covert actions to stop Khan’s black-market trades. Such efforts so far have been aimed at detecting and intercepting shipments of illicit materials. But Bush said Wednesday that intelligence and law enforcement officials needed to move beyond intercepting shipments and take direct action against men such as Khan where they live and work.

Some experts praised Bush’s speech Wednesday for elevating the importance of fighting nuclear proliferation, though they said his proposals were not dramatic.

“He forcefully presented and reinforced to the world the seriousness with which we take these issues,” said Leonard Spector, a nonproliferation expert at the Monterey Institute of International Studies. “But there’s not a whole lot there.”