"Operations at the Pantex Plant have paused until further notice. All weapons and special materials are safe and unaffected," plant operators said in a post on X, formerly Twitter.

Satellite imagery from the Amarillo National Weather Service showed the fires spreading near the northern city of Amarillo, spurred by strong winds and unseasonably warm temperatures.

The Texas A&M Forest Service reported 31 fires in the state. The largest blaze, known as the Smokehouse Creek Fire, had burnt 300,000 acres and was uncontained by late Tuesday evening.

The Pantex plant, located 21 miles (34 kilometres) from Amarillo, said in an earlier post there was no fire on the facility's site.

"All employees have been accounted for and non-essential personnel are no longer on-site," it said.

The Pantex facility in Texas assembles and disassembles the US nuclear stockpile, carries out special nuclear material testing and manufactures high explosives.

Amarillo's weather service warned people to stay indoors.

"Air quality remains poor in the Amarillo area... as smoke continues to stream southward," it said in a post on X.

Texas governor Greg Abbott issued a disaster declaration for 60 counties earlier Tuesday, freeing up fire response resources.

"Texans are urged to limit activities that could create sparks and take precautions to keep their loved ones safe," Abbott said in a statement.

Several towns, some up to 100 miles (160 kilometres) from Amarillo, issued evacuation orders, closing roads and ordering citizens to take shelter at home or in public facilities.