At least 23 people have been killed in the US state of West Virginia after extreme flooding destroyed homes, downed power lines and triggering mudslides. 

Among the youngest dead is a four-year-old boy, who was swept away after slipping and falling into rushing waters, a fire official told the AP news agency.

Local authorities declared a state of emergency in 44 of the state's 55 counties on Thursday. 

The unrelenting rain was described by the National Weather Service as a "one-in-a-thousand-year event". Between eight to 10 inches of rain fell in six to eight hours in some areas, according to the forecaster.

West Virginia Governor Earl Ray Tomblin said the flooding was "among the worst in a century for some parts of the state".

Emergency services and 200 members of the National Guard have been deployed to lead rescue efforts, while helicopters conduct search operations in the worst-hit communities. 

ON @GMA: Flooding and heavy rain sweep away homes and cars in West Virginia.https://t.co/qCNj7CWV23

— Good Morning America (@GMA) June 24, 2016