PHILADELPHIA, Sep 21 (Reuters) Two or three explosions and a fire in the basement of an office building rocked central Philadelphia today, leading to evacuation of about 5,000 workers but causing no injuries, officials said.

Workers for the PECO utility who were in a building in central Philadelphia reported the underground explosions, said Lisa Rotolo, a spokeswoman for the Philadelphia police.

Rotolo said there were no reports of any injuries and that some parts of the surrounding area had been sealed off by police. She said the cause was not immediately known.

A spokesman for the PECO utility said there was a fire in an underground cable vault but the cause was not immediately known.

''We are not exactly sure how that fire was caused,'' the spokesman said in an interview on KYW radio.

Philadelphia Fire Commissioner Lloyd Ayers said about 5,000 people were evacuated from two office buildings and that the fire was contained and primary electrical lines were unaffected.

Fox television news reported an apparent utility malfunction caused the explosion. CNN quoted the FBI as saying there was no apparent link to an extremist attack.

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