A heavy weight lowered by a National Guard Chinook helicopter has failed to break up an ice jam that's keeping water levels dangerously high on the Red River in North Dakota and Minnesota.

The pilot tried placing a 1,400-kilogram concrete weight on the jam near Oslo, Minn., on Sunday, but only a few pieces of ice broke free.

Officials will continue efforts Monday to break up the six-kilometre sheet of ice that is slowly moving toward the community.

The river is continuing its slow retreat after cresting late Saturday. The water has dropped below a record level to 11 metres, but officials have cautioned residents not to let their guard down, because the river is still nearly seven metres above flood stage.

Adding to the pressure to maintain a constant vigilance during the flood is the weather forecast. A new storm system is expected to bring up to 25 centimetres of snow to the waterlogged region Monday.

The snow is expected to melt quickly later this week, possibly pushing lakes and  ice-clogged rivers even higher.

There's a danger high winds from the storm will cause waves in the flood waters that will put more pressure on the sandbag dikes along the river.

Despite intense sandbagging efforts, water broke through a floodwall on Sunday and submerged a Lutheran school campus in Fargo, N.D.

The flood has been caused by an enormous winter snowfall that melted and combined with more precipitation to send the river to record levels.

The river flows from south to north through very flat terrain that presents few opportunities for drainage.