Heavy rains have swamped areas of the US northeast already sodden by last week's Hurricane Irene, causing flash floods and forcing thousands to evacuate early Thursday.

The National Weather Service issued a flash flood warning for several counties in New York, New Jersey and Pennsylvania, and Binghamton, New York ordered a mandatory city-wide evacuation.

"A flash flood warning means that flooding is imminent or occurring. If you are in the warning area move to higher ground immediately," the National Weather Service said in an early morning advisory.

The rains are the remnants of Lee, a storm system that slammed into the Gulf Coast on Sunday, generating high winds that have fanned devastating fires in drought-stricken Texas while dumping rain on the already-drenched northeast.

In Binghamton, authorities ordered a mandatory evacuation due to "imminent flooding" of the Susquehanna and Chenango Rivers.

"Residents able to drive are encouraged to go to a safe location. Buses will be in the area to pick up evacuees and take them to the Broome County Airport. Bring your medications with you," the city said on its website.

The flash flood warnings include much of New Jersey, which suffered devastating flooding from Hurricane Irene after it made landfall hundreds of miles (kilometers) south and trailed heavy rains all the way up the coast.