North Dakota breathed a slight sigh of relief Sunday as the ice-laden Red River floodwaters dropped a little, but officials warned that the waters remained at record highs and days of work remained ahead to keep the city of Fargo from being overwhelmed, dpa reported.

A dike burst behind Fargos Oak Grove Lutheran School Sunday, forcing the evacuation of 12 people and telling "us the fight is not over," Fargo's mayor Dennis Walaker was quoted as saying by Bloomberg news.

An early spring storm was expected to bring another 30 centimetres of snow and more subfreezing temperatures to parts of the region Monday, raising new worries of another flood crest when the snow melts.

Walaker was still worried that "theres a lot of water out there to the south of us." The river flows north into Canada.

Despite week-long subfreezing temperatures, volunteers worked through rain and snow to fill and stack 3 million sandbags to build up and strengthen dikes. But Walaker said workers could only relax when they've filled another 500,000 bags to hold in reserve.

"Are we feeling better? Yes, we are, but is it over yet? No," he said.

The Red River in Fargo and the neighbouring city of Moorhead had crested at 12.4 metres Saturday and dropped slightly, but still broke the record set in 1897 and surpassed severe flooding in 1997, the National Weather Service said.

The river remained more than 6 metres above flood stage.

More than 10,000 residents of the north central state's largest metropolitan area, home to more than 100,000 people, evacuated Friday. Two deaths are blamed on the floods, which left 62 people injured, according to a North Dakota Department of Emergency Services statement.

Forecasters warned the water level will remain high for many days and could yet rise up again. Subfreezing temperatures of minus 14 degrees Celsius appeared to have slowed runoff into the river.

US President Barack Obama on Saturday praised the thousands of volunteers who have filled sandbags to fight the rising waters.

"Even as we face an economic crisis which demands our constant focus, forces of nature can also intervene in ways that create other crises to which we must ... respond urgently," he said. "For the people of North and South Dakota and Minnesota who live along rivers spilling over their banks, this is one such moment."

Obama said federal officials were working with governors in each state to offer assistance. The Department of Homeland Security and the Federal Emergency Management Agency were coordinating Washington's response.