Beijing -- China has detected 30 cases of radioactive contamination in passengers, vessels and containers that have entered the country since March 16, quarantine authorities said on Friday.

The contamination cases were reported after China's quarantine bureaus started to monitor radiation levels at nine entry ports including Beijing, Liaoning and Zhejiang, said Li Yuanping, a spokesman for the General Administration of Quality Supervision, Inspection and Quarantine.

The first case involved a plane that arrived in the city of Dalian from Japan on March 16.

Other cases have been reported on several ships and containers, as well as in two Japanese tourists.

Li Yuanping said that quarantine authorities will continue to closely monitor radioactive levels at China's ports to ensure public safety.

On April 8, China banned imports of agricultural products and food from 12 regions located close to the site of Japan's Fukushima nuclear power plant, which was critically damaged during the recent earthquake and tsunami.

No abnormal levels of radiation have been detected in products imported from Japan so far, according to Liu.