Oklahoma towns hard hit by tornadoes begin clean-up after four killed in storms

A tornado caused extensive damage in Sulphur, Oklahoma (KOCO via AP)

Small towns in Oklahoma have begun a long clean-up after tornadoes flattened homes and buildings and killed four people, widening a destructive outbreak of severe weather across the middle of the US.

Punishing storms that began late Saturday in Oklahoma injured at least 100 people, damaged a rural hospital, washed out roads and knocked out power to more than 40,000 customers at one point, state officials said.