The plume of fumes from a flooded Texas chemical plant is "incredibly dangerous," the head of the US Federal Emergency Management Agency (FEMA) said Thursday.

Two explosions were reported overnight at the chemical plant in the town of Crosby and officials have ordered residents within 1.5 miles (three kilometers) of the facility to evacuate.

"By all means, yes, the plume is incredibly dangerous," FEMA Administrator Brock Long told a press conference.