(CNN) -- A levee breach along the swollen Missouri River prompted the evacuation Sunday of about 600 people in the area between the river and Hamburg, Iowa, officials said.
Residents were told they needed to leave within 24 hours.
The U.S. Army Corps of Engineers reported the breach occurred Sunday morning on a levee located in Atchison County, Missouri, near Hamburg.
Crews worked furiously to repair the breach, which Gen. Derek Hill, head of Iowa Homeland Security and Emergency Management, described as a boil, believed to be less than two inches in diameter.
He said the evacuation order was made as a precaution, in case the boil develops into a full-blown breach.
The National Weather Service issued a flash flood watch for southwest Iowa Sunday. If the levee fails, Interstate 29 and parts of Hamburg would likely be flooded, it said.
The cause of the breach was not immediately clear.
Crews were using a helicopter to pick up sandbags and fly them out over the levee, Hill said.
"No guarantees, but we certainly hope it will (work)," he said, noting the technique has been used before.
The levee breach follows weeks of high flows and record releases from dams in Montana and the Dakotas, according to the Corps. Heavy rains and snow pack runoff could result in near-record flooding along parts of the Missouri River this year.