Torrential rains across France forced thousands of people from their homes and saw stranded motorists rescued by soldiers as flood waters rose, while in Paris a metro line was shut and staff at the Louvre museum were told the venue was likely to close.

An 86-year-old woman was found dead in her flooded house in a small town southwest of Paris late on Wednesday, apparently the first casualty from the heavy rains that caused the Loire and Seine rivers to burst their banks.

President Francois Hollande declared a state of emergency in the worst affected areas and promised money to help local authorities deal with the flood damage.

"Since yesterday it's just been a deluge," said Jerome Coiffier, an inhabitant of Longjumeau, less than 20 km south of Paris, where firemen wading thigh-deep in water rescued inhabitants using inflatable boats.

Prime Minister Manuel Valls visited Nemours, 75 km south of Paris, where at least 3,000 out of 13,000 inhabitants were evacuated, as flood water crept towards the second story of buildings in the town centre. He called the situation "tense".

In the French capital, the Seine rose above 5 m (16 feet), forcing the SNCF rail operator to close an underground commuter line that runs along the river and is used by tourists to reach the Eiffel Tower and Notre-Dame Cathedral.

Meanwhile, Louvre museum chief Jean-Luc Martinez told employees in an internal email seen by Reuters that it was highly likely that the museum would have to shut and would then seek help from volunteers. The Musee d'Orsay said it would close earlier than planned.

In the Loire valley, the Chambord castle, a Unesco heritage site, found itself surrounded by water.

The national weather service said the greater Paris region had in May endured its wettest month since 1960.

In the Loiret region, where local officials called on the army to help evacuate motorists trapped on the A10 motorway, the floods are the most severe in 100 years.

In Paris, the Seine could peak at 5.5 m overnight or on Friday night, the environment ministry said in a statement. The river reached a record high of 8.60 m in 1910.  -Reuters