A water main break in Oakdale early Sunday morning caused a large washout under Interstate 694, and motorists will face a major detour "for days," said a spokesman for the Minnesota Department of Transportation (MnDOT).

After banning traffic on a two-mile stretch of I-694 in both directions all day on Sunday, MnDOT enlarged the portion that is closed, which now stretches from I-94 in the south to Hwy. 36 in the north.

All motorists will be guided to detours along I-94, I-35E and Hwy. 36 until the water main is repaired and the freeway is considered safe again, according to Kevin Gutknecht, MnDOT spokesman. He added that the affected stretch of freeway carries up to 87,000 vehicles per day.

The freeway will be closed "not for hours, but for days," he said Sunday.

The break occurred in a 12-inch diameter pipe connected to the municipal water supply system, said Oakdale police spokeswoman Michelle Stark.

The cause of the leak is not known, Gutknecht said, and the city is investigating.

The washout formed under the southbound lanes of the interstate just north of the 15th Street N. overpass and was visible in the median.

The interstate driving surface was not affected, Stark said. No one was injured, and no vehicles encountered the washed-out area, she added.

"We are asking the public to conserve water," she said. "The entire community will be impacted."

Workers were on the scene of the burst water main by about 6 a.m. Sunday, when a problem first was detected, Stark said.

A little less than three hours later, a resident nearby told police water was flowing from the leak.

A city official said efforts were underway to get the municipality's water tower refilled. He estimated that water pressure would get back to normal sometime Sunday afternoon.

Stark said there is "no time line" for repair of the break being completed but water was showing signs of stabilizing early Sunday afternoon.

Several neighbors near the overpass said they still had water at their homes, but they had noticed diminished pressure starting about 9:30 a.m.

At first, MnDOT shut down I-694 between 10th and 34th avenues north. That two-mile closure grew to about five miles with the bigger shutdown announced Sunday night. Gutknecht said the larger closure is needed so vehicles can be "detoured through like-sized roads."

By late Sunday, heavy equipment and workers were on the scene, as onlookers gathered on the 15th St. overpass.

Monday "is going to be a challenge for motorists on that side of the metro area," Gutknecht said, with much heavier traffic on I-35E to the west.

Star Tribune staff writers Erin Adler and Pat Pheifer contributed to this report. Paul Walsh • 612-673-4482