Two Facebook workers have possibly been exposed to sarin gas at a mailing warehouse at the social media giant's HQ in California, fire officials say.

The mail processing centre was evacuated as officials worked to determine what caused mail to test positive for the nerve agent.

"We have two possible exposures but right now they are not exhibiting any signs or symptoms," Menlo Park Fire Marshal Jon Johnston said.

Mr Johnston said incoming mail processed by a machine tested positive for the substance.

FBI agents are at the Silicon Valley site along with a hazmat crew, a spokesman said.


Facebook spokesman Anthony Harrison said four buildings were evacuated and that three have been cleared for people to go back in.

The suspicious package was delivered around 11am local time to one of the company's mail rooms, he said.

"Authorities have not yet identified the substance found," Mr Harrison wrote.

Sarin is a toxic compound that disrupts the nervous system and has been used as a chemical weapon.