Hurricane Nate is now a Category 1 storm and is picking up more strength as it barrels towards the US Gulf Coast, the National Hurricane Center said.

On Friday, the storm gained force as it sped past Mexico's Yucatan Peninsula after drenching Central America in rain that was blamed for at least 25 deaths.

Mitch Landrieu, mayor of New Orleans, declared a state of emergency, saying his city is preparing for the hurricane as it get stronger.

"I want to reiterate this to the people of New Orleans, we have been through this many, many times - there is no need to panic. We're encouraging the public to do all they can to prepare as we do in every weather event; you have to have a plan," Landrieu said.

"You have to prepare to protect your personal property. It is the cooperation of the citizens that make these events work really well with our great first responders."

Mississippi officials also declared a state of emergency and Louisiana ordered some people to evacuate coastal areas in advance of expected landfall on Saturday night or early Sunday.

Evacuations began at some offshore oil platforms in the Gulf.

Mississippi's government said it would open 11 evacuation shelters in areas away from the immediate coast, with buses available for people who cannot drive.

The US National Hurricane Center warned that Nate could raise sea levels by 1.2 to 2.1 metres from Morgan City, Louisiana, to the Alabama-Florida border. It had already had caused deadly flooding in much of Central America.

The centre added metropolitan New Orleans and Lake Pontchartrain to its latest hurricane warning.

The storm was located about 800km south-southeast of the mouth of the Mississippi River and was moving north-northwest at 35km/h.

Central America flooding

In Nicaragua, Nate's arrival followed two weeks of near-constant rain that had left the ground saturated and rivers swollen. Authorities placed the whole country on alert and warned of flooding and landslides.

Nicaragua's vice president and spokeswoman, Rosario Murillo, said that at least 11 people had died in that country due to the storm. Earlier Thursday, she had said 15 people had died before later revising to say some of those were still counted as missing.

She did not give details on all the deaths, but said two women and a man who worked for the Health Ministry were swept away by a flooded canal in the central municipality of Juigalpa.

Costa Rica's Judicial Investigation Organism blamed seven deaths in that country on the storm and said 15 people were missing. Flooding drove 5,000 residents into emergency shelters.

In Honduras, there were three dead and three missing, according to Oscar Triminio, spokesperson for the country's firefighters.

Damage caused by the storm prompted Costa Rican officials to postpone a World Cup qualifying football match between that country and Honduras, which had been scheduled for Friday night.

In Louisiana, Governor John Bel Edwards declared a state of emergency and mobilised 1,300 National Guard troops, with 15 headed to New Orleans to monitor the fragile pumping system there.

With forecasts projecting landfall in southeast Louisiana, Edwards urged residents to ready for rainfall, storm surge and severe winds - and to be where they intend to hunker down by "dark on Saturday".