An ExxonMobil pipeline leaked crude oil into the Yellowstone River near Billings, Montana.

An ExxonMobil pipeline leaked crude oil into the Yellowstone River near Billings, Montana. (Getty Images)

BILLINGS, Mont. -- The president of Exxon Mobil Corp's pipeline unit said the company still does not know the cause of a pipeline leak that has sent about 1,000 barrels of oil into Montana's Yellowstone River, but that it may change the way it conducts pipeline safety reviews.

"This will give us additional information to think about when we consider doing risk assessments on any line that has a river crossing anywhere in the country," Gary Pruessing, president of ExxonMobil Pipeline Co said during a Monday press conference in Laurel, Montana.

Exxon does not own any other underwater pipelines in the region, he said.

On Sunday, Exxon said it shut down a crude oil pipeline after as many as 1,000 barrels of crude oil leaked into the Yellowstone River in Montana. The oil company has estimated that from 750 to 1,000 barrels of oil were released before the leak was detected.

One barrel of oil is equivalent to 42 gallons (163 liters).

Exxon estimated the spill based on the six minutes it took from the time technicians detected an abnormal change in pressure to shutting the pipeline down, Pruessing said.

The leak followed a company shutdown of the pipeline in May after the city of Laurel had safety concerns due to the rising levels of the river from rain and runoff.

"At the time we shut down the line ... and went down and did a further risk assessment to make sure the site, based on the technical knowledge we had, was something we'd feel comfortable to run," Pruessing said. "We restarted the line feeling like we had a safe operation."

By Monday afternoon, Exxon had received 71 calls from nearby residents asking questions or reporting damage from the spill.

Although Exxon does not yet know the cause of the leak, Pruessing said it is investigating a theory that the fast-moving waters stripped the underwater dirt covering from the pipe, when then may have been struck by debris.

Pruessing declined to estimate the cost or how long it will take to clean up, which on Monday involved about more than 200 people.

Also on Monday, Montana Governor Brian Schweitzer said authorities would review the safety of all oil and gas pipelines that cross waterways in the state and close those that did not meet standards.

Pruessing declined to estimate the cost of spill and the related effects, saying it was too earlier to determine the extent of the damage.

In May, Exxon shut down segments of two pipelines in Louisiana due to flooding concerns, purged the oil and filled them with fresh water until the flooding ended.

In that case, the pipelines were not buried as deeply as those in Montana, which a December 2010 review showed were covered with five to eight feet of dirt. The water currents also differed.

"If we had had any anticipation of this kind of event here, that is exactly what we would have done," he said. "But again, as we did the risk assessment, we felt very comfortable from a technical standpoint that we did not have those same issues here."