Too many governments around the world don't believe terrorists can "go nuclear" and that complacency is slowing efforts to lock down the makings of atomic bombs, experts said Monday on the sidelines of President Barack Obama's nuclear security summit.

"There are a number of people inclined to think that maybe concerns about nuclear terrorism are alarmist, that terrorists could never make a functioning nuclear weapon," said former Australian Foreign Minister Gareth Evans, co-chair of an international nuclear study commission.

Evans and others, who staged an unofficial parallel "summit" of more than 200 specialists from dozens of countries, said they believe a crude nuclear weapon is within the capabilities of al-Qaida and other groups.

"It is possible, plausible and, over time, probable" that a "determined and well-financed" terrorist group will set off a nuclear blast somewhere, said U.S. nonproliferation expert and former Ambassador Robert Gallucci.

The assembled scientists, arms negotiators, policy scholars and others applauded Obama's hosting of the two-day summit of leaders of almost 50 nations, to rally support for his goal of securing all vulnerable nuclear materials worldwide within four years.

But the experts said the job would require a long-term commitment and leadership by the U.S. In particular, they urged the U.S. to ratify a 2005 treaty amendment requiring protection of its domestic nuclear materials.

"It's a remarkable fact that the host of this nuclear security summit hasn't gotten around to ratifying this amendment," said Harvard University's Matthew Bunn. "It's a little embarrassing."

Evans, familiar with the presummit negotiations among governments, said he expected Tuesday's final communique would focus on the need for more countries to ratify the amendment.

Pakistani physicist Pervez Hoodbhoy warned that Pakistan and India are building up their nuclear arsenals and questioned Pakistan's assurances that security is airtight for its bomb material.

"Unfortunately, I do not see this concern either in Pakistan or India about nuclear terrorism," he said. "Both countries do not see the seriousness of this situation."

Terrorists with a crude bomb wouldn't necessarily target American or European cities, Gallucci said. "Any country that has suffered serious terrorist attacks, foreign or domestic, needs to take this threat seriously."