Activists in four states were arrested after they cut padlocks and chains and entered remote flow stations to turn off valves.
Activists in four states were arrested after they cut padlocks and chains and entered remote flow stations to turn off valves in an attempt to stop crude moving through lines that carry as much as 15 percent of daily U.S. oil consumption. The group posted videos online showing the early morning raids.
Protest group Climate Direct Action said the move was in support of the Standing Rock Sioux Tribe, which has protested the construction of a separate $3.7 billion pipeline carrying oil from North Dakota to the U.S. Gulf Coast over fears of potential damage to sacred land and water supplies.
Officials, pipeline companies and experts say the protesters ran the danger of causing environmental damage themselves by shutting down the lines. Unscheduled shutdowns can lead to a build-up of pressure and cause ruptures or leaks, they said.
The activists had studied for months how to execute the shutdowns safely, said Afrin Sopariwala, a spokeswoman for the group.
"We are acting in response to this catastrophe we are facing," Sopariwala said, referring to global warming.
Police confirmed four arrests, three in Washington state and one in Montana. Protesters were also arrested in Minnesota and North Dakota, the activist group said, after the action early on Tuesday.
The pipelines carry crude produced from Canada's oil sands to the United States. Environmentalists have fought for years to stem Canadian oil sands production, which some call tar sands, in favor of cleaner energy.
Together, the lines affected can carry up to 2.8 million barrels of oil a day. No damage has been reported to the lines and the shutdown had little impact on the oil market. One of the lines restarted on Tuesday.
"Tampering with energy infrastructure is a dangerous activity and it could cause harm to citizens and surrounding communities, which is unacceptable," said Canadian Energy Minister Jim Carr.
His ministry was monitoring the situation closely, he said, adding that safety and security of energy infrastructure was a top priority.
Carl Weimer, executive director at the industry watchdog Pipeline Safety Trust, said the action was a "dangerous stunt".
"Closing valves on major pipelines can have unexpected consequences endangering people and the environment. We do not support this type of action," he said.
The incident is the latest in a series of actions by environmentalists and others in response to growing concern over the effects of fossil fuels on the environment and the potential effects on land and livelihoods of spills.
On Monday, 27 people were arrested for protesting the North Dakota pipeline including actress Shailene Woodley, who narrated her arrest on Facebook Live.