Mississippi - Several people were killed after drenching thunderstorms moved through Louisiana and Mississippi at the weekend, triggering flooding across the lower Mississippi valley, authorities said.

Rainfall, which meteorologists said reached two feet in some areas, killed three people in Louisiana and one in Oklahoma. Two fishermen were missing in Mississippi on Sunday, according to emergency management officials. President Barack Obama declared flooding in Louisiana a major disaster on Sunday, providing aid for victims.

Louisiana's emergency management office warned in a statement "the crisis is not over." It said some 5,000 homes had already been damaged.

The National Weather Service (NWS) warned on Sunday night of a tornado threat and potential for hail and damaging winds across eastern Arkansas and northern Louisiana.

Portions of the lower Mississippi Valley were at risk for severe thunderstorms through Monday morning, the NWS said, which could trigger flash flooding. River levels were expected to remain high in the region as excessive rain water drains, it said.

The Louisiana National Guard, working around the clock for several days, said it had rescued more than 3,295 citizens and 316 pets. Some 1,300 guardsmen responded to flooding in more than 25 parishes, conducting evacuations, search and rescue by vehicle, boat and helicopter, and providing security and shelter.

Authorities and meteorologists described the flooding as historic and the worst seen in the region apart from that spawned by hurricanes.

Scores of roads and bridges were closed throughout the region at the weekend. In Mississippi, almost 400 homes suffered damage from the rainfall and flooding, the state emergency agency said.

Meanwhile, cars from a passenger train derailed early Monday in the US state of Kansas, with some 20 passengers taken to the hospital but no initial reports of life-threatening injuries, rail carrier Amtrak said.

Two local hospitals that received passengers also confirmed that they were not treating any critical wounds, Wichita broadcaster KSN reported. The Los Angeles to Chicago Amtrak train, which derailed just after midnight some 20 miles (32 kilometres) west of Dodge City, was carrying 128 passengers and 14 crew members.

The Kansas Highway Patrol told US media that five cars were on their side. Amtrak said in a statement that those not at the hospital would be taken to a recreation center and provided alternate transportation.

The track on which the accident occurred is owned and operated by BNSF Railway, which Amtrak is working with to investigate the cause of the accident, the statement said. The Red Cross was at the scene providing assistance and emergency responders had provided on-site care, it added.