The US government's Los Alamos National Laboratory has declared an emergency after a fast-moving wildfire forced officials to close the nuclear site in New Mexico.
The fire has grown to 3,500 acres since it started about 12 miles south-west of the town of Los Alamos.
County authorities issued voluntary evacuation orders for both Los Alamos and White Rock, but between 50 and 100 rural homes were under mandatory evacuations.
The laboratory has activated its emergency operations centre and officials said all radioactive and hazardous material was being protected and the fire had not yet crossed on to laboratory property.
The site was founded during the Second World War to develop the world's first nuclear weapons.