An anthrax attack is a particular danger, the commission warns
The chance of a nuclear or biological attack on a major world city within the next five years is now much greater, a new report has warned.
A bi-partisan commission set up by the US Congress said America's "margin of safety" was shrinking, not growing.
One of the authors of World at Risk said its enemies were moving quickly to gain weapons of mass destruction.
The White House later said it did not plan to overhaul the country's national security structures in response.
A spokeswoman said it would be up to Barack Obama to consider the issue, when he took office.
"I think that we would make sure that the president-elect's team is fully briefed and then if they decide they want to move forward when they have their team together," said its spokeswoman, Dana Perino.
The report was presented to Vice-President-elect Joe Biden on Wednesday.
Biological threat greatest
Earlier, President George W Bush discussed the report with the nine members of the Commission on the Prevention of Weapons of Mass Destruction Proliferation and Terrorism, along with his National Security Adviser, Stephen Hadley.
The commission, which was set up after the attacks of 11 September 2001, warns in its report that without urgent action, "it is more likely than not that a weapon of mass destruction will be used in a terrorist attack somewhere in the world by the end of 2013".
Barack Obama is urged to appoint a new official to oversee the threat
The threat to the US and the rest of the world, the report says, is from the rapid spread of nuclear technology in countries such as Pakistan and Iran, and poor security in biotech industries worldwide.
It says that the threat from biological weapons is greatest, adding that the US should be less concerned that terrorists will become biologists and far more concerned that biologists will become terrorists.
One of the members of the commission, former Democratic Senator Bob Graham, said that the threat was growing because America's adversaries were moving at a faster pace to get access to weapons of mass destruction.
He said time was not America's ally and that it needed to move with a sense of urgency.
The commission wants President-elect Barack Obama to appoint a senior official to co-ordinate intelligence to combat the spread of nuclear and biological weapons.
Pakistan is highlighted as a country of particular concern. The report says there is a grave danger is could become "an unwitting source of a terrorist attack on the United States, possibly with weapons of mass destruction."
It also accuses the Bush administration of failing to treat possible biological attacks with the same priority as the spread of nuclear weapons.