A slow-moving storm is forming into a likely hurricane in the Gulf of Mexico, and is forecast to hit the already-flooded US city of New Orleans.

The storm, which is currently classified as a tropical storm, has already brought thunderstorms to the city, triggering flash floods.

A state of emergency is in effect and residents in some low-lying areas have been asked to evacuate.

The river may come dangerously close to overtopping levees shielding the city.

The Mississippi River may rise as high as 20ft (6m) above sea level, officials warn, potentially exceeding the 20-25ft levees protecting New Orleans.