The crash of a fire-fighting C-130 aircraft in South Dakota over the weekend killed four crew members and injured two others, the US military said Tuesday.

The Air National Guard released the casualty toll and announced its fleet of specially-equipped planes were cleared again for flights, after having been grounded for a day to review safety measures following Sunday's crash.

The C-130 went down while battling a woodland fire in the southwestern state of South Dakota.

The fire-fighting cargo planes have been in heavy use this month amid a spate of wildfires, including the worst in Colorado's history.

Blazes in Colorado have burned through more than 100,000 acres (40,000 hectares) and authorities estimate they won't be fully contained until July 12 at the earliest.