A dozen people were missing at sea off the US state of Louisiana on Wednesday after a commercial ship capsized in the Gulf of Mexico, sparking a rescue operation that saved six others.

The United States Coastguard and a fleet of volunteers were deployed to search for the 18 crew on board after the vessel capsized in a storm near Port Fourchon, about 160 kilometers (100 miles) south of New Orleans.

The ship was a 129-foot (39.3 meters) lift vessel, the coastguard said, adding that the search for the remaining crew was ongoing.

"We're still looking for others right now," Petty Officer Jonathan Lally, a spokesman for the coastguard, told The Washington Post early on Wednesday.

The operation came as an unexpectedly strong storm hit the area, creating dangerous conditions for boats in the Gulf. 

The National Oceanic and Atmospheric Administration reported gusts up to 75 miles per hour (120 kilometers per hour) on Louisiana's southern coast, according to local media.