Two dozen police cars, nine fire engines and five ambulances were rushed 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 and no victims were immediately reported.

"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.

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