Halliburton has lost a seven-inch radioactive rod somewhere in the Texas desert. The National Guard has been called in to help to find the device, which employees of the controversial US oilfield services company lost a week ago.

The rod, which contains americium-241/beryllium and is stamped with a radiation warning symbol with the words "Danger Radioactive: Do not handle. Notify civil authorities if found", was lost during a 130-mile journey between oil well sites in Pecos and Odessa last Tuesday.

The US Nuclear Regulatory Commission (NRC) warned that the radioactive materials "could cause permanent injury to a person who handled them".

The agency said americium-241/beryllium, known as Am-241, is a "category 3" source of radiation and would normally have to be held for some hours before causing health problems.

But the NRC still warned that "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".

A spokeswoman for the NRC said there are no records of radioactive rods lost in the last five years. "[There has] never been one lost in the public domain [in the last five years]," she said.

"Someone wanting to blow this source up would have to find it first."

The route the Halliburton truck took between Pecos and Odessa has been painstakingly searched with radioactive detection gear three times with assistance from local police and the National Guard.

"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," the NRC said in its report into the incident. "The crew returned to the well site near Pecos and searched for the source, but did not find it. The radiation safety officer stated that the lock was found in the storage compartment in the back of the truck. The transport container plug was not in the container."

The three-man Halliburton crew, who had been using the rod to identify oil and gas deposits suitable for fracking, have been questioned by the FBI.

The NRC said Halliburton was carrying out a forensic search of the truck. "They are literally stripping it down, removing every piece of equipment looking for the source," the agency said.

Halliburton said it would offer a reward to anyone who finds the rod, but cautioned the public to stay at least 25ft away from the device.

"The route between the two well sites continues to be combed using specialised equipment in extensive ground searches and aerial analysis," the company said.

Halliburton, which was once run by former vice president Dick Cheney, has previously attracted controversy for its role in BP's Deepwater Horizon oil spill disaster, building Guantánamo Bay and for working in Iraq, Iran and Libya.