An urgent search was underway in Western Australia on Saturday after a tiny radioactive capsule was reported to have gone missing while being transported from a mine.

The 8mm by 6mm silver capsule, which is used in mining operations, has been unaccounted for since mid-January, emergency services said.

It contains the radioactive substance Caesium-137, according to Western Australia's Department of Health.

The capsule is believed to have fallen off a truck while being moved to a storage facility.

Health officials have warned it can cause radiation burns or sickness if handled.

Fire and emergency services said the capsule was lost between the remote town of Newman and the suburbs of northern Perth, a distance of about 1,400 kilometres (870 miles).

Officials have warned the public not to handle the capsule if found and to contact emergency services.

"The concern is someone will pick it up not knowing what they are dealing with," said Dr Andrew Robertson, chief health officer for Western Australia.

A fire service spokesman said they are prioritising populated areas during the search, which could take weeks.

Authorities were only alerted on Wednesday after the company responsible for the capsule realised it was missing, he added.