The head of the UN's nuclear watchdog, Mohammed ElBaradei, has warned of a "race against time" to stop terrorists procuring nuclear materials.
ElBaradei said terrorists could get their hands on nuclear materials
The head of the International Atomic Energy Agency was speaking at a US conference hosted by the Carnegie Endowment for International Peace.
He endorsed the influential think tank's new arms control plan.
Under the plan, major nuclear powers would be expected to make concessions in the interests of global security.
The IAEA director warned there was a real danger of uranium or plutonium falling into the wrong hands.
"We are actually having a race against time which I don't think we can afford," he said.
"The danger is so imminent... not only with regard to countries acquiring nuclear weapons but also terrorists getting their hands on some of these nuclear materials, uranium or plutonium.
"So the sooner that we start, the better for everybody involved."
The nuclear watchdog chief's message was picked up by the US Senator Sam Nunn, a security expert.
Mr Nunn told the BBC that the security of nuclear material in Russia was a key concern.
He said the biggest challenge was to have US President George W Bush and Russian President Vladimir Putin put the issue to the top of their agenda.
Mr Nunn was instrumental in last month's unveiling of a multi-million dollar initiative to stop extremist groups from building so called "dirty bombs" with nuclear material.
Governments around the world are becoming increasingly concerned about nuclear proliferation particularly since the revelations, in February of this year, that the Pakistani nuclear scientist AQ Khan had passed on nuclear secrets to a number of countries.
One of the authors of the Carnegie Endowment's plan, Joseph Cirincione, said the world was at "a nuclear tipping point".
The BBC's diplomatic correspondent in Washington, Jonathan Marcus, says the Carnegie plan is certainly ambitious in scope.
It argues that all current nuclear arms control problems need to be put into a single pot and handled together.
Everyone - both the nuclear haves and have-nots - have to be seen to make concessions if all are to gain.
But our correspondent says other experts in Washington are not so sure.
Political capital, they say, is limited and needs to be focused on individual proliferation, problems like that between India and Pakistan or the continuing uncertainties surrounding Iran's nuclear ambitions.