The ancient Iranian city of Bam was hit by a mild earthquake almost four years to the day after a catastrophic quake there killed tens of thousands of people, state media reported on Wednesday.

The suburbs of Bam in southeastern Iran were rocked by the 4.2 magnitude quake at around 8:00 pm (1630 GMT) on Tuesday evening, the state-run IRNA news agency reported.

The quake that hit Bam in the early hours of December 26, 2003 killed 31,000 people, about a quarter of the city's population, and destroyed the city's ancient mud-built citadel.

Iran sits astride several major faults in the earth's crust, and is prone to frequent earthquakes, many of them devastating.

Work is continuing in Bam to rebuild destroyed infrastructure as well as the citadel (Arg-e Bam), a UNESCO world heritage site that before the quake was one of the country's prime tourist draws for its well preserved mediaeval town.