As many as 10,000 residents were evacuated a day earlier from neighborhoods nearest the Souris, which cuts through the heart of Minot. It wasn't immediately clear how many more people were affected by Thursday's advisory.
National Guard Capt. Dan Murphy said officials were examining maps and planned to release more information at an afternoon news conference.
"The bottom line is they're just trying to get everybody out of the area where they think the property is going to be inundated,'' Murphy said.
Swollen by heavy rains and snowmelt far upstream, the Souris has risen rapidly since the weekend. On Thursday, officials accelerated the release of water from the Lake Darling dam and said that could raise the river 2 to 3 feet higher than earlier projections.
Officials also announced the closure of the Broadway Bridge, shutting down a key north-south artery in the city. Major traffic jams were reported Thursday afternoon and officials asked residents not to travel north unless of an emergency.
National Guard members checked pumps and added sandbags to the levee that protects the sewer and water treatment plant on the southwest side of the city.
The Souris had been expected to peak Sunday or Monday several feet above its historic high in 1881. On Thursday, that estimate was moved higher and earlier — to about 6½ feet above the record, with the peak sometime Saturday or Sunday and lingering for several days.
The river, which begins in the Canadian province of Saskatchewan and flows for a short distance though North Dakota, was all but certain to inundate thousands of homes and businesses during the coming week. Yet crews had not entirely given up.
Earlier Thursday, trucks and loaders carried clay and dirt to waiting Bobcats that sped to and fro, spreading and tamping the material atop riverside levees that already reached some 15 feet high. The workers and Guard members were the only people to be seen in the area.
Parts of the city were already flooding. One trailer park near the river was under several feet of water.
Besides raising levees, Lt. Col. Kendal Bergmann said the U.S. Army Corps of Engineers is letting more water out from Lake Darling now so that later releases don't have to be as big.
Before the Broadway Bridge closed, many people were using it as a sightseeing perch — some to check on their own homes.
Jodine Blake, 45, watched as water approached her two-story house, which stood out among others with its orange paint. She had moved some belongings to the second story in the hope they would be safe there.
"It just makes you cry. You lose everything,'' she said.
Dave Vander Vorste, 55, helped residents in two of his rental houses move out of the evacuation zone. He said he went through the historic flood of 1969 — which was eclipsed Thursday — and knows what lies ahead.
"It's going to be five days of shock followed by reality,'' he said.