As Hurricane Gustav lashed down heavy rain and wind on Louisiana on Monday, US faith-based development agencies say they are on standby to reach the needy with emergency supplies.

On Sunday, more than one million Americans streamed out of New Orleans, still recovering from the devastation wrought by Katrina when it killed 1,800 people and flooded 80 per cent of the city three years ago.

The city's mayor Ray Nagin ordered a mandatory evacuation of the city, warning that Gustav was the "mother of all storms" and "not the one to play with".

Christian relief and development agency World Vision has assembled emergency supplies at Picayune, Mississippi, and stationed several truckloads of goods in Dallas, Los Angeles and other cities.

"We're making the most of this early warning time to prepare," said John Pettit, director of World Vision's domestic disaster response, last Friday.

"Our staff in Mississippi and Texas have been contacting dozens of churches and community groups that we've partnered with since Katrina to let them know we want to stand by them for Gustav and help them serve the most vulnerable in their communities."

Currently a category three storm, Hurricane Gustav has already killed 80 people in the Dominican Republic, Haiti and Jamaica.

"We're hoping and praying that Gustav spares the Gulf Coast, but we know from experience that we have to be ready for a worst-case scenario," said Pettit, whose team has been actively coordinating with regional authorities and other members of the National Voluntary Organizations Active in Disasters (VOAD) in preparation for the storm's landfall.

Salvation Army personnel in Hattiesburg, Mississippi, have meanwhile set up a major staging area to support New Orleans in Gustav's aftermath and prepared 19 shelters across Texas. Specialist response equipment and 170 mobile feeding units remain on standby across the region.

Major Gene Hogg, divisional secretary for Alabama, Louisiana and Mississippi, warned residents on Saturday to get out of the way of the storm.

"This is shaping up to be a big storm and we are anticipating a long-term recovery effort will be needed. The more people we can get out of the way before it hits, the better."

Liberal documentary filmmaker Michael Moore, meanwhile, drew criticism after saying that the timing of Hurricane Gustav was "proof that there is a God in heaven".

"To just have it planned at the same time, that it would actually be on its way to New Orleans for Day One of the Republican convention, up in the Twin Cities, at the top of the Mississippi River" is proof of God's existence, he said to MSNBC's Keith Olbermann.

In response, Louisiana Congressman Steve Scalise released a statement Saturday blasting the documentary filmmaker for the remarks.

"I demand an immediate apology from Michael Moore to the people of south Louisiana for his offensive and inappropriate comments," said Scalise, a Republican.

"People in Louisiana, regardless of political affiliation, are making plans to leave to protect their families from this serious storm, and the God I know would not share Michael Moore's glee for our plight."

The hurricane has forced Republican presidential hopeful John McCain to scale back plans for the first day of the Republican convention in St Paul, Minnesota. US President George W Bush was due to deliver an address but will instead travel to Texas in order to keep up with the emergency response.