Work to upgrade Rome's public transport has again run up against an ancient problem: archaeological ruins.

Culture ministry officials on Monday showed reporters where work on the city's third subway line unearthed barracks for Roman Praetorian guards dating from the second century.

While construction workers poured concrete at the planned Amba Aradam metro stop, an archaeologist just a few meters away brushed dirt from a small bronze bracelet.

Archaeologists have also found what appears to be a collective grave at the barracks. Archaeologist Romina Mosticone says she has found 13 adult skeletons and a bronze coin.

Officials hope to incorporate the discovery into the new metro station, which is scheduled to open in 2020. Work on Rome's Metro Line C has been beset by delays since launching in 2007.