Homeland Security Secretary Michael Chertoff said these new regulations could eventually be expanded to include radiation scans at points in Europe and Asia for private planes flying to the US.

Private aircraft have not received the intense screening given to US-bound cargo and commercial flights.

'I have spent three years talking about people putting nuclear bombs in containers and cargo ships. And we have now got to where we basically scan all those containers,' Mr Chertoff said Wednesday in question-and-answer session with reporters. 'But in thinking about what I would do if I were a terrorist ... it occurred to me that why would I put the bomb in the container, if I can rent the jet and put it in the jet?'

The new security measures will dramatically reduce this risk, he said.

The regulations will have the biggest impact on private flights from Europe and Asia, because most of the 400 private planes that fly to the US each day are from those continents, Mr Chertoff said.

The US also is working with Canada to put similar rules into place for flights arriving there.

Mr Chertoff said his department may also work with Latin American countries in the future so that a nuclear bomb could not be flown into those countries on private planes.

'We want to first address that threat which would come in through the shortest and most direct route,' he said. 'And once we've got that dealt with, we'll need to think about more indirect routes.' -- AP