One witness said the blast was caused by a bomb hidden in a rubbish bin in the hallway of a large tower block on the floor shared by the three international companies.

"It was at 9.15 a.m. (0445 GMT) outside the BA and BP office, there were no casualties but the building was damaged," said the witness who works in one of the offices.

"I was sitting at my desk and suddenly we heard an explosion and there was smoke everywhere," he said. "Everyone panicked, especially the BA office people who are mostly women, they started to scream and we ran out of the office."

Iran is locked in a tense stand-off with Britain, Germany and France over it nuclear program. Iran said on Monday it had begun the process of restarting some nuclear activities which the three countries have warned could lead to UN sanctions.