(CNN) -- An ExxonMobil pipeline leaked an undetermined amount of crude oil into the Yellowstone River near Billings, Montana, prompting a burgeoning cleanup effort, officials said Saturday.

"It's significant. People are concerned," said Duane Winslow, director of disaster and emergency services in Yellowstone County.

But he expressed confidence Saturday afternoon that ExxonMobil would clean the river.

"They seem to be committed to it," Winslow said.

ExxonMobil Pipeline Company, a subsidiary that operates the line, established a claims phone number for those affected by the rupture.

"We deeply regret this incident has happened," said ExxonMobil spokesman Kevin Allexon. "The magnitude of this response will reflect that."

The pipeline runs under the river.

About 80 people from a regional response team were on site Saturday and 70 additional trained individuals were en route, Allexon said.

Crews placed absorbent pads and booms into the flooded river. Winslow was hopeful the high stage of the flooded river would mean more of the oil and contaminants would be washed ashore.

Heavy rains and a melting snowpack have caused the river to run high. The Yellowstone River was at 13.51 feet Saturday evening; flood stage is 13 feet.

The oil was carried by a pipeline from fields at and near the Wyoming-Montana border to Billings.

The cause was under investigation. Allexon said no information was available Saturday on the pipeline's age, depth under the river and its maintenance record. The spokesman said he could not speculate on whether river conditions had anything to do with the incident.

The affected pipeline segment was shut down.

"Any spill is significant in our eyes," Allexon said.

Between 200 and 300 residents near Laurel, where the spill was reported, were evacuated for a time late Friday and early Saturday, Winslow told CNN, because of the leak's odor. They were allowed to return home Saturday.

Laurel is about 15 miles west of Billings.

The leak was noticed around 11:30 p.m. Friday and the 12-inch pipeline was shut down within a half hour, Winslow said.

It was not clear Saturday evening how far the leak had stretched along the Yellowstone; Winslow said it affected about 70 miles in the county. Allexon had no estimate on the total length.

"The parties responsible will restore the Yellowstone River," Gov. Brian Schweitzer said, adding state agencies were helping to monitor the containment and cleanup.

The U.S. Environmental Protection Agency sent a team to the site. ExxonMobil, which is leading the incident command, is getting assistance from Clean Harbors, a provider of environmental services.

Laurel resident Jim Swanson, looking at oil near his home, told CNN Billings affiliate KULR, "I am just kind of reeling right now."