In the weapons cache were hundreds of Katyusha rockets and mortar shells as well as several thousand light arms, the report said, adding that the weapons had all been manufactured in neighbouring Iran.
The weapons were discovered in the citys al-Hora district, it said.
Um Qasr, the countrys largest deepwater port city, is close to the city of Basra which Iraqi officials say has become a hub for agents of Irans Ministry of Intelligence and Security (MOIS).