Hurricane Isaac hits Gulf Coast, headed for New Orleans
Hurricane Isaac has come ashore in south-eastern Louisiana and is heading towards New Orleans.
Isaac is lashing coastal areas with winds of up to 80mph (130km/h) and it is expected to reach New Orleans exactly seven years after it was hit by Hurricane Katrina.
The city has closed the new floodgates designed to protect it from flooding.
President Barack Obama warned residents in Isaac's path to heed evacuation warnings and not "tempt fate".
Thousands of people have fled the area; many of those who stayed behind have locked themselves indoors.
At the scene
The wind has started to build up and the rain lashing down in the French Quarter of New Orleans as the narrow, high-sided streets is amplifying the wind speed. Debris is starting to be tossed around on Canal Street, as roadworks are being pulled apart by the high winds and trees shaken almost from their roots.
Some areas have lost power already. The streets have been cleared of all but the foolhardy - and the journalists with TV satellite trucks parked in the lee of buildings.
The timing of the storm has brought so much attention to this city devastated by Hurricane Katrina when it struck seven years ago.
I sat through the storm then in a hotel room and watched as 4x4s were pushed along roads with the force of the wind, wrought iron staircases ripped off buildings and windows were blown out.
The wind speeds so far are just a third of those during Katrina but it's the volume of water falling and the storm surge being driven onshore which threatens homes and those who have decided to sit out the storm.
Many people have chosen to stay - boarding up their doors and windows and settling in for a long night, hoping Hurricane Isaac will be kinder to their city.
At 00:00 EDT (04:00 GMT) the centre of the hurricane was estimated to be 65 miles (105km) south of New Orleans and moving at about 8mph (13km/h), the US National Hurricane Centre (NHC) said.
The BBC's Alastair Leithead in New Orleans says roadworks are being pulled apart by winds and trees are being shaken from their roots.
Power cuts have been reported across low-lying parts of Louisiana and Mississippi, affecting more than 200,000 homes and business.
Mr Obama has declared an emergency in Louisiana and Mississippi, allowing federal funds to be released to local authorities.
"I want to encourage all residents of the Gulf Coast to listen to your local officials and follow their directions - including if they tell you to evacuate," Mr Obama said on Tuesday.
Speaking from the White House, he added: "Now is not the time to tempt fate. Now is not the time to dismiss official warnings. You need to take this seriously."
Shortly before Isaac reached hurricane status on Tuesday, Louisiana Governor Bobby Jindal called for the Federal Emergency Management Agency (Fema) to make a full emergency declaration for the state.'Great wall' of New Orleans
He told reporters that a previous declaration did not allow for the reimbursement for state's expenses from the storm.
"We have learned from past experiences that you cannot wait and you have to push the federal bureaucracy," said Mr Jindal, who cancelled an appearance at the Republican National Convention because of the storm.
New Orleans Mayor Mitch Landrieu said his city was "officially in the fight" on Tuesday, as he confirmed its airport was closed and would not serve as a shelter.
Mr Landrieu said that a 26ft (8m) high levee gate that now protects the areas of the city that were badly flooded in 2005 - which he dubbed "the great wall of New Orleans" - was closed on Tuesday morning.
"We will not have a Katrina-like event," he said, adding there will still be parts of the city that will likely be flooded.
"Do not let this storm lull you into complacency," he said. "People may be getting bored. It's better to be bored than to get hurt."
Officials have not ordered any evacuations, telling residents to reinforce their homes and stock up on supplies instead.'Storm surge'
The bowl-shaped city of New Orleans is particularly vulnerable to storms, with the centre of the city the furthest below sea-level.
Latest weather updates from National Hurricane Center
Red Cross searchable website for disaster shelters
List of area curfews from local broadcaster WWFL
Emergency alerts from the City of New Orleans
Evacuation map from the New Orleans Times-Picayune
New Orleans power outage map
Residents are hoping that billions of dollars spent on reinforcing flood defences that failed catastrophically in 2005 will hold this time.
Isaac has killed at least 24 people in Haiti and the Dominican Republic, and caused significant flooding and damage in the Caribbean.
It largely bypassed the Republican convention in Tampa, Florida, but prompted a day-long delay to proceedings there.
The National Hurricane Center warned that a possible combined "storm surge" and high tide would cause flooding in coastal areas along the Gulf Coast.
Water would potentially reach 6-12ft (1.8-3.7m) above ground in south-west Louisiana and Mississippi, 4-8ft in Alabama and 3-6ft in south-central Louisiana.
Isaac is also threatening heavy rainfall of as much as 20in (51cm) in isolated spots, and could spark possible tornadoes along the northern Gulf Coast.