The pipeline carries Canadian heavy crude oil from Patoka, Illinois to refineries on the Texas Gulf coast.
Exxon Mobil issued a release saying the company was responding to a spill of more than 10,000 barrels, and that some 4,500 barrels of oil and water had been recovered.
The latest spill comes at time when proponents of the proposed Keystone XL pipeline have been trying to convince Washington to give the $7 billion project the green light.
Opponents of TransCanada Corp.'s plan to pipe Alberta oilsands bitumen to the U.S. Gulf Coast denounce it as an environmental catastrophe in the making.
Dan Gatti of the advocacy group Environment America says the Arkansas spill provides a glimpse into what would happen if the Keystone pipeline got built.
"These images of oil-coated disasters need to become reminders of a bygone era, not a sign of things to come," Gatti said in a statement Sunday.
Exxon said the 20-inch pipeline had been shut down as crews tried to prevent the spilled oil from reaching a nearby lake.
It said cleanup operations were being co-ordinated with the Department of Emergency Management and other local authorities, and that the cause of the spill was being investigated.
Last Monday, federal regulators proposed that Exxon Mobil pay $1.7 million in civil penalties for safety violations linked to a pipeline rupture that spilled an estimated 238,000 litres of crude oil into Montana's scenic Yellowstone River in July 2011.
The spill fouled approximately 110 kilometres of the Yellowstone River's banks, killing fish and wildlife and prompting a massive, months-long cleanup.
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