HOUSTON (AFP) - Floodwaters have breached a levee south of the city of Houston, officials said on Tuesday (Aug 29), urging residents to leave the area immediately.

"The levee at Columbia Lakes has been breached!!" the Brazoria County government tweeted on its official feed.

"GET OUT NOW!!"

The Columbia Lakes area lies south of Houston, large parts of which remain underwater four days after monster storm Harvey made landfall on the US Gulf Coast, triggering widespread flooding.

A statement on the official Brazoria County website said that County Judge Matt Sebesta "has issued a MANDATORY EVACUATION for all residents living WEST of State Highway 288 and SOUTH of State Highway 6."

The statement said there was only one way out of the area.

"THERE ARE NO OTHER EVACUATION ROUTES OUT OF BRAZORIA COUNTY," the statement said, adding that one state highway "WILL NOT be open indefinitely and will be unpassable with additional rains and river flooding.

"LEAVE NOW!"

The statement directed evacuees to a shelter at the Expo Centre in Bell County - about 370km to the north-west - adding: "Pets are welcome!"