An ExxonMobil pipeline in Montana State ruptured late Friday, which spilled hundreds of barrels of crude oil into the famed Yellowstone River, prompting evacuations of nearby residents.

The company said the pipe had been shut down and the segment where the leak happened had been isolated. Precisely how much oil leaked into the river was still unclear.

ExxonMobil released the following update as cleanup operations continued Sunday evening following a release of oil into the Yellowstone River.

More than 120 people were involved in the response and cleanup effort including ExxonMobil's North America Regional Response Team, the Clean Harbors oil spill response organization, and additional contractors. An additional 80 people were expected on site Monday.

For the purposes of the response, the area downriver of the spill has been organized into four zones. Cleanup activities are focused in the first two zones, Laurel to Duck Creek Bridge, a distance of seven miles from the spill location, and Duck Creek Bridge to Johnson Lane (12 miles). Reconnaissance and evaluation activities are under way in the second two zones, Johnson Lane to Miles City (144 miles) and Miles City to Glendive (78 miles).

Cleanup crews are using absorbent pads to soak up oil, boom to isolate oil that has pooled adjacent to the river and vacuum trucks and tankers to pick up and dispose of the oil.

Air quality monitoring is ongoing and has confirmed no danger to public health. Municipal water systems are being notified to monitor water quality but no reports of impacts have been received to date.

Daily aerial flights over the river are being undertaken to locate additional oil locations and monitor and direct cleanup activity.

Workers from the International Bird Rescue are expected to arrive on Monday. The Montana Audubon Conservation Education Center and Yellowstone Valley Audubon have offered to provide wildlife recovery services and facilities. One report of impacted wildlife has been received.

Contacts with senior government officials were made throughout the day. ExxonMobil is coordinating the response with the Environmental Protection Agency; the Montana Department of Environmental Quality; U.S. Department of Transportation Pipeline and Hazardous Materials Safety Administration; Montana Fish, Wildlife and Parks; Yellowstone County Disaster and Emergency Services; and Yellowstone County commissioners.

The Exxon leak started sometime late Friday, with crude oil having traveled 80 miles downstream by 1 pm local time and in some places settling on the shore line, Custer County Disaster and Emergency Services Coordinator James Zabrocki told Wall Street Journal.

Nearby residents in Laurel, Montana, were evacuated in the wee hours of the morning but were able to return to their homes by 6 am, a spokesman for Laurel City Fire and Ambulance told Reuters. Evacuation orders for all of Yellowstone County have been lifted, the emergency services spokesman told Reuters.

Throughout Saturday, clean-up crews worked to lessen the impact of the spill, laying down absorbent material along the banks of the river in Billings and near Laurel to mop up some of the escaped oil, and measuring fumes to determine the health threat.

The accident happened from a 12 crude pipeline operated by ExxonMobil Pipeline Company that runs from Silver Tip, Montana to Billings, Montana. All appropriate state and federal authorities have been alerted, the company said in a statement.

An estimated 750 to 1,000 barrels of oil had leaked from the pipe for about a half-hour before it was shut down, ExxonMobil spokeswoman Pam Malek told Associated Press. Other Exxon officials estimated up to 158,982 litres of crude oil had escaped.

Oil has been found as far as five miles down the river from the pipeline location and additional reports of oil sightings are being investigated.

We are bringing in experts from across the country to clean up the oil, ExxonMobil Pipeline Company president Gary Pruessing said in Billings, where he is leading the response effort. We will stay with the cleanup until it is complete, and we sincerely apologize to the people of Montana for any inconvenience the incident is creating.

Exxon Mobil said no cause has been identified for the release of oil from the pipeline, which met all regulatory requirements and has undergone inspection most recently in December. A field audit of the pipeline's integrity management program was undertaken by U.S. Department of Transportation Pipeline and Hazardous Materials Safety Administration in June.

Pipeline pumps were shut down within seven minutes of a pressure loss and action to isolate the pipeline was immediately initiated. The amount of oil released is estimated to be between 750 and 1,000 barrels.

Air quality monitoring throughout the impacted area is ongoing and has confirmed no danger to public health. Municipal water systems are being notified to monitor water quality but no reports of impacts have been received to date.

Crews responded from ExxonMobil's Billings refinery Saturday, and used boom and absorbent pads to pick up oil and staged response equipment throughout the area. An additional 50 people trained in oil spill response were expected to join the effort today. They will be joined by members of ExxonMobil's North American Regional Response team from across the United States who have expertise in oil spill emergency response operations.

We recognize the seriousness of this incident and are working hard to address it, said Pruessing. We will continue to add resources and are extremely grateful for the patience and assistance of local residents and authorities.