Media playback is unsupported on your device Media caption US Army Corps of Engineers spokesman Col Ed Fleming: "Public safety is our number one priority" when opening the Morganza Spillway

US army engineers have opened floodgates in Louisiana that will inundate up to 3,000 sq miles of land in an attempt to protect large cities along the Mississippi River.

It is hoped the move on Saturday will ease pressure on Baton Rouge and New Orleans.

This is the first time in four decades the level of the Mississippi has forced the floodgates to be opened.

About 25,000 people and 11,000 buildings could be adversely affected.

Fed by rainwater and the spring thaw, the Mississippi and its tributaries have caused massive flooding upstream, and officials have said the flooding in Louisiana is the worst since 1927.

The US Army Corps of Engineers warned that if the spillway was not opened, New Orleans could be flooded by about 20ft (6m) of water.

Two other gates - the Birds Point-New Madrid Floodway and the Bonnet Carre Spillway - were opened earlier this month.

'Not a sprint'

The Morganza Spillway, 45 miles (72km) north-west of Baton Rouge, was last opened in 1973.

It stands above the Mississippi's normal water level, and comes into play only when the Mississippi is already swollen and endangering the surrounding areas.

By opening its floodgates engineers are able to control the flow of the floodwaters, diverting them around Baton Rouge into the Atchafalaya river basin, a low-lying area of central Louisiana.

Morganza Spillway Image caption The Morganza Spillway allows water from the Mississippi River to divert into the Atchafalaya Basin Built in 1954 to relieve flood pressure on Mississippi River

Last opened in 1973

20 miles (32.2km) long

125 gates release up to 600,000 cubic feet/sec (17,000 cubic metres/sec)

Water will flow south, flooding homes and farms in the state's Cajun country under an expected 10-20ft of water.

Over several days, the water should run south to Morgan City - where workers are rushing to reinforces levees - and then into the Gulf of Mexico.

Corps spokesman Col Ed Fleming said: "It's a historic day, not only for the entire Mississippi River but for the state of Louisiana".

"Today's the first day in the history of our nation that we have had three floodways open."

Col Fleming said the opening would be slow to "make sure folks have the understanding that water is coming their way and they evacuate according to their local procedures".

Wildlife also needed time to get to higher ground, he said.

Opening all 125 gates on the spillway would release 600,000 cubic ft of water every second.

It doesn't make us feel any good that [by] protecting New Orleans, other folks are going to get hurt Mitch Landrieu, Mayor of New Orleans

Just one bay was opened on Saturday, allowing 10,000 cubic ft of water per second to pass. Within 30 minutes, 100 acres of land were under a foot of water, the Associated Press reported.

One or two more gates are expected to be opened on Sunday and then others if needed.

Col Fleming said the main water crest was not expected at the spillway until 24 May and would last for 10-14 days, so that "no doubt that structure has the potential to be opened for the better part of three weeks".

"We are here with the communities fighting these floods shoulder to shoulder."

Maj Gen Michael Walsh added: "The crest is still up in Arkansas. It's a marathon, not a sprint - there is huge pressure on the system as we work the water through. The protection of lives is the number one thing we're looking for."

The current flooding is approaching records set 84 years ago, when hundreds of people in the region died.

Image caption Three-thousand square miles (7,770 sq km) are expected to be submerged by the planned flooding. In pictures: Louisiana braces for floods

The trigger for the spillway opening was when 1.5m cubic ft (42,500 cubic metres) of water per second was flowing down the Mississippi River at Red River Landing, just north of the spillway.

Col Fleming said he was optimistic for Morgan City, as the walls are 20ft and the crest is expected at 12ft.

But Mayor Tim Matte said some areas had no protection and were "fighting a battle almost on a house-by-house basis".

One man in the town told reporters he had had his housed raised from 2ft to 4ft off the ground in an attempt to save it.

"This is our life savings here, but it's worth every penny," said Michael Grubb. "How could we leave our home?"

'Pack everything'

The American Red Cross has prepared shelters for thousands of people who are fleeing the region.

And while some are angry that their region has been effectively sacrificed to secure Baton Rouge and New Orleans, other residents say they have lived for years with the knowledge that if and when the next flood came, it would come their way.

New Orleans Mayor Mitch Landrieu said although he believed his city would be safe, this was a "tragic situation" for those in the Atchafalaya basin and Morgan City.

"Our hearts go out to them. It doesn't make us feel any good that [by] protecting New Orleans, other folks are going to get hurt."

Residents of the town of Butte La Rose, directly in the path of the spillway's water, said they had been told to pack for a long absence.

"They told us to move as though we were moving - period - not coming back, not to so much as leave a toothpick behind," said one woman.

Farmers in the region are expecting to lose their entire crops in a year of high prices for farm produce.

"The land's going to wash away, but that's life," said Hurlin Dupre, from Krotz Springs in the Atchafalaya river basin.

"The worst of it is we are in a drought and we can't use none of that water."