The US government is not doing enough to thwart attempts by terrorists to acquire nuclear weapons, members of the former 11 September inquiry have said.
The group warns reports of detainee abuse are damaging to the US
The US must also improve its image abroad, damaged by reports of abuse of terror suspects, the group said.
It was reporting on the government's progress in meeting key recommendations made by the 9/11 commission last year on how to prevent new terror attacks.
The pressure group was formed after the 9/11 commission was disbanded.
Called the 9/11 Public Discourse Project, the bipartisan body consists of the same commissioners that investigated the 11 September attacks.
In their report they said "minimal" or "insufficient" progress had been made on many of the 9/11 commission's key recommendations.
Some praise was given for US efforts to crack down on global terror financing.
The group also said "good progress" had been made in encouraging Muslim nations to integrate into global trade.
It called on President George Bush to make thwarting arms proliferation "his top national security priority".
"Preventing terrorists from gaining access to weapons of national security must be elevated above all other problems of national security," the group said.
There were particular concerns about the security of nuclear materials in Russia, said the group's chairman Thomas Kean - who also headed the 9/11 inquiry.
An agreement between the US and Russia in February which gave US weapons inspectors greater access to Russian nuclear sites was a step forward, he said, but not enough to contain the risk of material going astray.
"The most striking thing to us is that the size of the problem still totally dwarfs the policy response," said Mr Kean.
"We have no greater fear than a terrorist who is inside the US with a nuclear weapon."
He added that al-Qaeda had sought weapons of mass destruction for years, and had said it was willing to use them.
The group also criticised the Bush administration's efforts to improve its global image, tarnished by reports of the mistreatment of terror suspects.
The government "should work with its allies to develop mutually acceptable standards for terrorist detention", it said.
Lee Hamilton - who was co-chairman of the disbanded commission - added that "detainee abuse in Abu Ghraib, Guantanamo and elsewhere undermines America's reputation as a moral leader".
Mr Bush has previously defended his government's treatment of detainees, denying claims of torture and insisting "any activity we conduct is within the law".
The 9/11 commission's report in July 2004 urged sweeping changes to how the intelligence services operated, after finding the government had "failed to protect American people" from the 11 September 2001 attacks.