Serbia has flown 6,000 uranium rods - enough to make two nuclear bombs - back to Russia, in high-security operation that alarmed residents in the capital Belgrade.

We were vigilant and ready to cope with any potential assailant including [Osama] Bin Laden himself

Serbian police officer

Helicopters hovered over the city and special police in gas masks guarded the route from the Vinca nuclear research institute to a local airport, as the transportation operation began early on Thursday.

The Serbian Science Minister, Dragan Domazet, said city residents were in no danger, stressing that the operation had to be kept secret because of a risk of terrorist attack.

"You always have to foresee everything, like a possible terrorist attempt to seize the fuel. The quantity shipped was enough to make at least two atom bombs," he said.

He added that "these bars are completely harmless until they burn in a reactor".

'Vigilant'

A Serbian police officer told the Associated Press news agency that transportation of the rods to the Belgrade airport took hours and "was the most difficult for us in years".

"We were vigilant and ready to cope with any potential assailant including [Osama] Bin Laden himself," the police officer said.

The former Soviet Union gave the nuclear fuel to the Vinca institute in the 1970s for research purposes.

But the rods had been kept unused since the early 1980s, after a reactor at the institute was closed.

The removal of the nuclear fuel was organised in co-operation with the International Atomic Energy Agency and paid for by the United States.

The uranium rods have taken to the Ulyanovsk Nuclear Processing Plant in central Russia.