Vietnamese authorities are searching for a lead box containing hazardous radioactive material which has gone missing from a steel factory, an official said Wednesday.

The box of cobalt-60, which has a wide range of uses including for radiotherapy and in industry, has disappeared from the Vietnamese-owned Pomina steel mill in the south of the country.

"(We) do not know how and when the container went missing," Do Vu Khoa, an official with the Department of Science and Technology in the southern province of Ba Ria-Vung Tau, told AFP.

"We are searching for the radiation box," he added.

The silver-white container weighs 45 kilograms (100 pounds) and is some 1.5 feet long (45 centimetres) and six inches wide, the department said in a statement.

"It contains Co-60 which was used for liquid measurement. It poses a potential danger to the environment and people's health," the statement said.

State-run Tuoi Tre newspaper said the cobalt-60 was among five radioactive sources which Pomina imported in 2010 to measure liquid steel levels at its plant.

The equipment was last confirmed to be at the facility late last year, the company said, according to the report.

The search zone includes several waste dumps in Ba Ria-Vung Tau and surrounding areas including Ho Chi Minh City.

"It's our top priority to look for the container," Mai Thanh Quang, director of the science department, was quoted as saying.

The biggest risk is that a scrap collector could find the box and cut open the protective lead casing, potentially exposing himself and others to radiation, he said.

Cobalt-60 is a radioactive isotope of the metallic element cobalt and the gamma rays it emits destroy tumours.

Apart from radiotherapy, it can be used to irradiate food and sterilise health care products.

But direct contact or mere proximity can cause cancer without proper safeguards.