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Officials have begun a controlled release of water at a Mississippi dam that was battered by Hurricane Isaac.

Tens of thousands residents are being evacuated in Louisiana downstream of the barrier at Lake Tangipahoa, which is damaged but remains intact.

Already having weakened to a tropical storm, Isaac was downgraded further to a tropical depression late on Thursday.

But forecasts say it could continue to bring heavy rain as it moves north into the drought-hit central US.

The storm has left hundreds of homes flooded in its wake while passing over Louisiana

Residents with boats have been plucking people from the roofs of their homes.

Up to 50,000 residents are being evacuated in south-eastern Louisiana downstream of the impaired dam at Lake Tangipahoa. There are fears the barrier could fail, flooding an already swollen river.

One of the hardest-hit areas was Plaquemines, near New Orleans, where waters overflowed a levee on Wednesday, causing widespread flooding.

Hundreds of homes were evacuated, but many residents stayed, believing they could ride out a far weaker storm than Hurricane Katrina in 2005.

Louisiana Governor Bobby Jindal has ordered buses to the area to help evacuate residents, Tangipahoa Parish President Gordon Burgess told local broadcaster WWL.

In the town of Amite, Louisiana, Police Chief Jerry Trabona said officers were going door-to-door along the Tangipahoa River.

More than 7,000 National Guard troops, the US reserve armed forces, have been called into service across four states, the majority in Louisiana.

Gov Jindal activated all Louisiana guardsmen on Wednesday, which would bring total forces in the state to more than 8,000. They have helped rescue or evacuate more than 3,000 people in the state since the storm hit.

By 16:00 local time (21:00 GMT) on Thursday, Isaac had weakened further to a tropical depression with maximum sustained winds of 35mph (55km/h), .

But forecasters say Isaac could still dump up to 14in (35cm) of rain over much of Louisiana, Mississippi, south-west Alabama and Arkansas through to Friday. Some areas could see rainfall of up to 25in, the NHC said.

The storm pummelled New Orleans on Wednesday, on the seventh anniversary of Hurricane Katrina, which devastated the region and killed at least 1,800 people in the Gulf Coast area.

Defence systems built to protect the city passed their first major test, according to the US Army Corps of Engineers.

By Thursday morning, more than one million residents of Louisiana and Mississippi were without power due to Isaac, according to the US Department of Energy.

Isaac is expected to move further inland over the next several days before breaking up over the weekend.

The storm killed at least 24 people as it passed over Haiti and the Dominican Republic earlier this week.