Two dozen police cars, nine fire engines and five ambulances deployed to the scene of the fire at the factory run by US firm Edge Autonomy on the outskirts of Latvia's capital, Riga.

The cause of the blaze was not known but two people were taken to hospital, Latvia's emergency services tweeted, adding a third person was receiving treatment at the facility site.

"A high-risk fire has occurred in the production building, sparking a lot of smoke," the fire service said on Twitter, urging local residents to keep their doors and windows closed.

Hours after some 50 firefighters were dispatched to the scene rescue services reported the blaze was under control as police launched an investigation into the incident.

The California-based company produces long-range unmanned aircraft for intelligence, surveillance and reconnaissance missions, according to its website.