Troops deployed in New Orleans

Tropical Storm Isaac strengthened into a hurricane just off the US Gulf Coast today, lashing the New Orleans area with strong winds and heavy rain seven years after the city was devastated by Hurricane Katrina.

Isaac's storm surge could pose a major test of New Orleans' new flood control systems and reinforced levees. Forecasts from the US National Hurricane Center showed the storm coming ashore in the Mississippi Delta late tonight, possibly taking direct aim at the so-called Crescent City.

"Isaac has finally formed into a hurricane, so we are officially in the fight and the city of New Orleans is on the front lines," New Orleans Mayor Mitch Landrieu told reporters.

"Citizens have to be prepared. I'm going to ask you to hunker down," Mr Landrieu said, as hundreds of US Army National Guard troops took up strategic positions around New Orleans.

Brandishing automatic assault rifles to ward off any threat of looting, the troops in military vehicles took up positions on mostly deserted streets. Their arrival came as driving rain and stiff winds began battering the city's iconic French Quarter and its boarded-up storefronts.

Earlier, the Army Corps of Engineers closed for the first time the massive new floodgate on the largest storm-surge barrier in the world, at Lake Borgne, east of New Orleans.

In other preparations, oil production in the U.S. Gulf of Mexico ground to a near halt, and ports and coastal refineries curtailed operations as Isaac neared the Louisiana coastline.

At 2 pm local time (6pm Irish time), the Hurricane Center said Isaac was centered about 217 km southeast of New Orleans with top sustained winds of 120 kph.

Its forward speed was a relatively slow 16 kph, a concern for people in the path of the storm since slow-moving cyclones can bring higher rainfall totals. The storm was about 595 km wide.

Isaac spared Tampa, Florida, where the Republican National Convention began yesterday. But it forced party leaders to revamp their schedule and they may have to make further revisions so as not to be seen celebrating Mitt Romney getting the party's presidential nomination while Gulf Coast residents are struggling through the storm.

President Barack Obama added his concerns in a statement from the White House, saying: "We're dealing with a big storm and there could be significant flooding and other damage across a large area."

Now was "not the time to tempt fate," he added, saying people should heed warnings and evacuate if instructed by authorities to do so.

Isaac had New Orleans in its sights as the city still struggles to recover from Katrina, which swept across it on Aug. 29, 2005, killing more than 1,800 people and causing billions of dollars of damage.

After Katrina, the Corps of Engineers built a $14.5 billion flood defense system of walls, floodgates, levees and pumps designed to protect the city against a massive tidal surge like the one that swamped New Orleans in Katrina's wake.

The floodgate that closed today is 26 8 meters high and 2.9 km long. It was designed to prevent the Inner Harbor Navigation Canal from breaching its walls, as it did in 2005, inundating the Lower Ninth Ward, Gentilly and New Orleans East neighborhoods, and St. Bernard Parish.

Most of the Lower Ninth, which still shows the devastation of Katrina, was deserted and quiet by lunchtime today. Residents who hadn't evacuated were unloading water, food and fuel from their cars and trucks into their homes.