US prosecutors have charged a man from the state of Pennsylvania and two foreign citizens with conspiring to illegally export chemical weapon detection devices and other equipment to Syria, according to authorities.

A federal judge on Wednesday unsealed the case after the prosecutor, Todd Hinkley, signed a plea agreement for Harold Rinko, of Pennsylvania, one of the three men who were charged in 2012.

Rinko, along with Ahmad Feras Diri, of London, and Moawea Deri, a Syrian citizen, stand accused of illegally shipping goods to Syria through other countries for nine years by creating false invoices, mislabelling items and listing fake purchasers and end users.

"We know they were exported to Syria," Hinkley said. "The end user information we weren't able, at least to this point, to develop in the investigation."

Prosecutors said the items included masks used in civil defence against chemical agents, industrial engines used in oil and gas fields, a device used to locate buried pipelines and portable instruments used to detect, measure and classify chemical agents.

The United Arab Emirates, Jordan and the UK were allegedly used as transfer points for the items.

Unanswered questions

A 31-page indictment did not say how authorities discovered the alleged scheme, and Hinkley said that he could not comment on it.

Investigators also do not know who used the products once they were in Syria.

Diri and Deri allegedly used Rinko as a "front" to purchase the items and ship them to countries without export bans.

Prosecutors said Diri is awaiting extradition in London, where he was arrested last year.

Deri is thought to be in Syria, which has no extradition treaty with the US.

Syrian opposition activists and other witnesses told the AP news agency that government forces have attacked rebel-held areas with poisonous chlorine gas in recent months.

But the Syrian government denies these claims.