image sourceEPAimage captionNew Orleans residents who had not left the city by Sunday were preparing to ride out the storm

Hurricane Ida has made landfall in the US state of Louisiana, with winds of 150mph (240km/h) and a potentially "catastrophic" storm surge, the National Hurricane Center (NHC) says.

Thousands of people fled the state ahead of the storm, but by Sunday morning it was too late to leave.

It could be the worst hurricane to hit Louisiana since the 1850s, state Governor John Bel Edwards said.

Ida gathered strength over the warm waters of the Gulf of Mexico.

It made landfall near Port Fourchon, south of New Orleans, as a category four hurricane on the five-step Saffir-Simpson scale – meaning it will cause severe damage to buildings, trees and power lines.

In some places the storm surge could be as high as 16ft (4.8m), potentially submerging parts of the low-lying coastline.

The impact of climate change on the frequency of storms is still unclear, but increased sea surface temperatures warm the air above, making more energy available to drive hurricanes.

As a result, they are likely to be more intense with more extreme rainfall.

Ida made landfall on the 16th anniversary of Hurricane Katrina, in 2005, a category three storm that inundated New Orleans, killing more than 1,800 people.

The US National Weather Service (NWS) warned New Orleans residents to shelter in place immediately, if they had not already done so. "Go to an interior room or a small room with no windows. Stay put during this time," the NWS tweeted.

Louisiana hospitals are already under pressure from Covid-19. The state has the third-highest rate of infections in the US.

Normally, hospitals in the predicted path of the hurricane would be evacuated, but this time there are few beds available, even at facilities further inland.

"We don't have any place to bring those patients. Not in state, not out of state," Mr Edwards said.