Snow storms closing in on North Dakota on Sunday threatened more misery for residents sheltered behind miles of dikes, as teams shored up strained levees holding back record flood waters.

One floodwall protecting a Fargo school cracked early Sunday, flooding the campus, and sending teams of police and firefighters to the scene to repair the breach.

But officials warned blizzards and strong winds forecast for later Sunday could whip up waves as high as two feet which may wash over the city's defenses, holding back walls of water from the swollen Red River.

Authorities fear as many as 30 000 people could be left homeless in the northern plains if the river breaks through levees protecting North Dakota's largest city, Fargo, as well as Moorhead lying on the opposite bank in Minnesota.

The weary region had been granted a few hours relief Saturday as the water levels sank due to the extreme cold halting the snow melt, and the level was holding steady Sunday at 40.3 feet (12.21 meters). But city officials warned the raging river was not done with them yet.

"With water this high we absolutely are in the watch and respond and plug mode," Fargo mayor Dennis Walaker said.

The Red River is putting enormous amounts of pressure on the city's 77 kilometers of protective dikes and levees and crews are struggling to reinforce weak spots and contain minor leaks, he told reporters.

Warnings posted on the state's emergency website said a blizzard watch had been issued from Sunday afternoon until Monday night for southwest and central Dakota, bringing strong winds of 40 kilometers to 72 kilometers an hour.

"Easterly winds associated with this storm may cause waves of up to two feet on top of the crest which will impact the western dikes along the Red River. There is still a possibility the river could rise again," the website said.

That pressure is likely to continue for days, if not weeks, as the floods make their way slowly northward to Canada and are replenished with inflows from tributaries and overland flooding.

Teams of volunteers were patrolling the levees and the temporary sandbag dikes overnight.

"We simply need to have people patrolling the streets, watching the dikes and calling us when they see something," Walaker said.

Thousands of people have already fled their homes as this flat prairie state remained blanketed with snow and flooded waterways which were closing in on isolated farms and smaller towns.

Water levels in some homes had reached the second floor while small dikes kept others dry in the middle of a deep, muddy lake.

Bitterly cold temperatures may have saved the city from a deluge by freezing some of the floodwater and preventing further melting, officials said.

Downtown Fargo was largely empty after the mayor asked non-essential businesses to close their doors and residents to stay off the roads to make room for rapid response teams.

Sandbagging efforts were set to resume Sunday morning at the Fargodome stadium where desperate volunteers have already filled more than 2.5 million bags used to build a series of dikes.

Earthen levees were also being reinforced and officials were considering using helicopters to lower enormous bags of sand into the floodwaters at key points.

Officials said the river at Fargo reached a high of 12.4 metres early on Saturday – breaking the previous record of 12.2 metres in 1897 – and only inches below the top of Fargo's highest levees.

Curt Kesselring, who has lived through four of Fargo's floods and lost his home to two of them, was digging in for a long fight against the slow-moving water.

"I think we've crossed (the worst of) it but there can be plenty of things you can't predict," Kesselring said as he picked up better-fitting waders and a fresh pair of waterproof gloves at a local shop.

"You've got to keep an eye on the pumps, and you've got to be going out and checking the dikes all the time."

Both Grand Forks, North Dakota and Winnipeg, across the border in Canada, have flood diversion systems which should protect them from the forecasted crests.

AFP