Exxon Mobil was working to clean up thousands of barrels of oil in Mayflower, Arkansas, after a pipeline carrying heavy Canadian crude ruptured, a major spill likely to stoke debate over transporting Canada's oil to the United States.

Exxon shut the Pegasus pipeline, which can carry more than 90,000 barrels a day of crude oil from Pakota, Illinois, to Nederland, Texas, after the leak was discovered on Friday afternoon, the company said in a statement.

Exxon, hit with a $1.7 million fine by regulators this week over a 2011 spill in the Yellowstone River, said a few thousand barrels of oil had been observed.

A company spokesman confirmed that the line was carrying Canadian Wabasca Heavy crude. That grade is a heavy bitumen crude diluted with lighter liquids to allow it to flow through pipelines, according to the Canadian Energy Pipeline Association, which referred to Wabasca as "oil sands" in a report.

22 homes evacuated

The US Environmental Protection Agency has categorized the incident as a major spill, indicating a release of more than 250 barrels of oil, ExxonMobil said in the statement.

The company confirmed the existence of "a few thousand" barrels of spilled oil in the area.

No one was injured, but about 22 homes in the affected area were evacuated, the company said.

Numerous crews were working on Saturday to clean up the spill and prevent it from reaching nearby Lake Conway, officials said on Saturday.

Approximately 4,500 barrels of oil and water have been recovered, Exxon said.

It would be a "number of days" before those evacuated were allowed to return to stay, said Faulkner County Judge Allen Dodson, according to the website Arkansasonline.com.

"That's an ongoing battle," Dodson said. "The rain hinders us in terms of our ability to work and in terms of the water flow," he said.

It's still unclear what caused the underground pipeline to burst, Dodson said.

"It's just really too early to speculate," Dodson said. "I think Exxon has said that it's too early for them to go and open up the dirt and look. That may take some time to determine."

Second incident

The spill occurred as the US State Department is considering the fate of the Keystone XL pipeline, which would carry crude from Canada's oil sands to the Gulf Coast. Environmentalists, concerned about the impact of developing the oil sands, have sought to block its approval.

Supporters say Keystone will help bring down the cost of fuel in the US.

The Arkansas spill was the second incident this week where Canadian crude has spilled in the US. On Wednesday, a train carrying Canadian crude derailed in Minnesota, spilling 15,000 gallons of oil.

Exxon expanded the Pegasus pipeline in 2009 to carry more Canadian crude from the Midwest to the Gulf Coast refining hub and installed what it called new "leak detection technology".

Exxon said federal, state and local officials were on site and that it was staging a response for a spill of more than 10,000 barrels "to be conservative".

Reuters-Xinhua