(Adds details, comment) MELBOURNE, Jan 30 (Reuters) - Rio Tinto...

(Adds details, comment)

MELBOURNE, Jan 30 (Reuters) - Rio Tinto Ltd's (RIO) iron ore division chief Simon Trott apologised on Monday after a contractor hired by the mining giant lost a radioactive capsule in transit in Western Australia, sparking a radiation alert across parts of the state.

It is unclear how long the radioactive capsule, part of a gauge used to measure the density of iron ore feed, has been lost. It left Rio's Gudai-Darri mine site on Jan. 12 and Rio said it was told by the contractor that the capsule was missing on Jan. 25.

As of Monday, the radiation alert remained in place. Authorities said it was lost during transport from north of Newman - a small town in the remote Kimberley region - to a storage facility in the northeast suburbs of Perth, a journey of 1,400 kilometres (870 miles) miles or further than the length of Great Britain.

A search is now underway for the small silver capsule which measures 6 millimetres (mm) in diameter and is 8 mm long and contains Caesium-137 which emits radiation equal to 10 x-rays per hour. Authorities have recommended people stay at least five metres (16.5 feet) away.

"We are taking this incident very seriously. We recognise this is clearly very concerning and are sorry for the alarm it has caused in the Western Australian community," Trott said in a statement.

"Rio Tinto engaged a third-party contractor, with appropriate expertise and certification, to safely package the device in preparation for transport off-site ahead of receipt at their facility in Perth," he said, adding that Rio was also conducting its own investigation into how the loss occurred.

Prior to the device leaving the site, a Geiger counter was used to confirm the presence of the capsule inside the package, Rio said. Authorities believe that it fell through a hole in the truck after a container collapsed as a result of vibrations.

"We have completed radiological surveys of all areas on site where the device had been, and surveyed roads within the mine site as well as the access road leading away from the Gudai-Darri mine site," Trott said.

Analysts said that the transport of dangerous goods to and from mine sites was routine, adding that such incidents have been extremely rare and did not reflect poor safety standards on the part of Rio.