Facebook employees may have been the target of a sarin attack, according to reports.

NBC News reports a machine at the company's mailing facility in Menlo Park, California, alerted employees to the possibility of a package containing sarin.

Fire authorities said no employees have been exposed to the substance.

Sarin is a "human-made chemical warfare agent classified as a nerve agent," according to the US Centre for Disease Control and Prevention.

It's a highly toxic, clear and odourless liquid that can disrupt the functioning of the nervous system. It has the ability to change from liquid to gas and when it does it can spread into the environment.

People are exposed to sarin via skin contact, eye contact or by breathing it in. It can also be mixed in water and food.

Facebook checks all of its mail and packages for dangerous substances, it's understood the suspicious package was detected about 11am local time.

A company spokesman told NBC that four buildings in the area were evacuated due to the threat.

Tech companies have been on high-alert in terms of security in recent years.

Last year a woman shot three people at YouTube's headquarters, the shooter was allegedly angry at YouTube's policy towards her video content.

Facebook was forced to evacuate staff late last year after a bomb threat was called in. The threat turned out to be false.

A fire department source reportedly told Business Insider that sarin was detected in a mail bag during a routine test, however detection may be a false positive.

The FBI has responded to Facebook's request for the hazmat team.

Severe exposure to sarin can result in loss of consciousness, paralysis or respitory failure leading to death.

In 1995 more than 5000 people fell ill following a sarin attack by the Aum Shinrikyo cult, who targeted trains heading to the Tokyo's government district in Japan.

In 2013 Syria was accused of using sarin to kill 150 people in the country, months later more than 1000 were killed, including hundreds of children, in an alleged chemical weapons attack near Damascus.

More to come.