A three-man crew from Halliburton lost a radioactive rod in the Texas desert (Reuters)

A radioactive rod has been lost in the Texan desert by an oil exploration crew.

Police and the US National Guard have launched a hunt for the seven-inch stainless steel radioactive rod, after the team, who were working for Halliburon, reported that they had lost it in transit through the desert.

"It's not something that produces radiation in an extremely dangerous form but it's best for people to stay back 20 or 25ft," said health department spokesman Chris van Deusen.

Stamped with the radiation warning symbol and the words, "Danger: Radioactive", the rod contains americium-241/beryllium (Am-241), a neutron-emitting source used in common electronic devices such as smoke detectors and medical diagnosis equipment.

The US Nuclear Regulatory Commission (NRC) said that it would normally take several hours of exposure for Am-241 to cause health problems. "It could possibly - although it is unlikely - be fatal to be close to this amount of unshielded radioactive material for a period of days to weeks," it said.

The FBI cleared the three-man crew who lost the rod during a 130-mile truck journey between the oilwell sites in Pecos and Odessa.

The rod is used to identify oil deposits suitable for initiating exploratory drilling.

"When the crew went to remove the Am-241 source they discovered the source transport container lock and plug were not in place and that the source was missing," a report from the NRC said.

It was earlier reported that Halliburton had offered a reward for retrieval of the rod but the company refused to confirm that or make any comment at all on the situation.

Halliburton is one of the largest oilfield service companies in the world.