Council workers in southern Spain have accidentally discovered a well-preserved necropolis from the Phoenician era with at least eight subterranean limestone burial vaults and a staircase.

Archaeologists said the "unprecedented" Phoenician-Carthaginian cemetery dates back to the fourth or fifth century BC.

Workers came across the site as they were improving the water supply system in the Spanish town of Osuna, in southern Seville province, where ruins of the Roman city of Urso exist.

The Andalusian regional government's Department of Culture and Historical Heritage said archaeologists discovered "a series of remains of unquestionable historical value" that due to their degree of conservation have been "unprecedented in inland Andalusia".

Excavation work is currently underway in order to reach the ground levels of a possible atrium, officials said.

The Spanish town of Osuna came to the spotlight when it became the location for parts of the fifth season of HBO series, Game of Thrones.