A South Australian man has been critically injured after being struck by a falling tree branch.

The 45-year-old is believed to have been helping a neighbour trim a tree in Sandy Creek, near Gawler, when a branch fell and trapped him.

He suffered head and chest injuries.

READ MORE: New anti-COVID-19 drug eliminated virus in three days, study shows

READ MORE: Fourth victim of tragic Victorian helicopter crash identified

The South Australian Country Fire Service attended to help free the man.

He was treated on the scene by paramedics before being airlifted to the Royal Adelaide Hospital.