The two children, a nine-month-old boy and his two-year-old sister, were in a car with both of their parents, their brother and their grandmother, when a creek in Bucks County, Pennsylvania, burst its banks.

The father and the four-year-old son "miraculously" managed to escape the flood water.

But the mother, the grandmother, and the two young children were swept away along the creek.

The mother was later found dead - one of five people killed in the flooding - while the grandmother survived.

A 100-strong search crew and numerous drones are looking for the siblings along the creek - which drains into the Delaware River.

The fire chief of Upper Makefield Township, Tim Brewer, said the search effort would be a "massive undertaking".

The family are all from Charleston, South Carolina, and were visiting relatives and friends when they got caught in a flash flood on Saturday, Mr Brewer said.

"As they tried to escape the fierce floodwaters, dad took his four-year-old son while the mother and the grandmother grabbed the two additional children," the fire chief said.

"However, the grandmother, the mother, and the two children were swept away by the floodwaters," Mr Brewer added.

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Flash flooding also hit parts of New Hampshire, Massachusetts, Connecticut, New York and New Jersey over the weekend.

The governor of New Jersey, Phil Murphy, declared a state of emergency on Sunday. He plans to tour damaged areas in the state.

A tornado touched down on Sunday morning in North Brookfield, Massachusetts, but no injuries were reported.

In New Hampshire, a Nascar race at the New Hampshire Motor Speedway was postponed on Sunday due to the flooding.

There were also delays and cancellations at airports in the New York City area over the weekend.