WASHINGTON Nov 10 (Reuters) - The U.S. Department of Justice on Friday pledged to prosecute protesters who damage energy infrastructure in a move that could escalate tensions between climate activists and the administration of President Donald Trump.

The DOJ said it was committed to vigorously prosecuting those who damage "critical energy infrastructure in violation of federal law." Attempts to "damage or shut down" pipelines deprive communities of services, and can put lives at risk, cost taxpayers millions of dollars, and threaten the environment, a department official said in a statement sent to Reuters.

The statement was in response to a letter sent last month to Attorney General Jeff Sessions by 84 U.S. Representatives asking whether domestic terrorism law covers activists who shut oil pipelines in October last year. The DOJ said it was reviewing the letter.

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The DOJ did not say whether it would investigate or prosecute the protesters who broke fences in four states last year and twisted shut valves on several pipelines importing crude from Canada that carry the equivalent of as much as 15 percent of U.S. daily oil consumption. The group Climate Direct Action said at the time the action was in support of the Standing Rock Sioux Tribe, which has protested Energy Transfer Partners LP's Dakota Access Pipeline.

Last month's letter to Sessions, spearheaded by Representative Ken Buck, a Colorado Republican, and signed by at least two Democrats, said that when a protester burns a hole in an operating pipeline it risks igniting the contents and "killing not only the perpetrator but other innocent victims." The letter had no details on any protesters who had ever actually taken a blowtorch to a live pipeline.

Five of the protesters, who say they only turned off valves on the pipelines, responded to Buck this week in a letter saying that their actions were nonviolent and were the "last resort in a desperate and necessary effort to avert catastrophic climate change."

States brought charges against the protesters. Ken Ward, who closed an emergency valve on a pipeline bringing oil sands crude from Canada, was sentenced to 32 days with most of it to be spent in community service.

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The DOJ's statement on the protesters is an escalation of the department's stance from the days of the administration of former President Barack Obama, a Democrat. In 2016, the DOJ along with the Army Corps of Engineers gave a temporary victory to the Standing Rock Sioux Tribe by refusing to approve an easement for Dakota Access. Trump moved to open Dakota Access early in his presidency.

Angst over pipeline projects could escalate later this month. Nebraska regulators are set to decide then whether TransCanada Corp's Keystone XL pipeline, a project that has galvanized environmentalists but which Trump supports, should go forward. (Reporting by Timothy Gardner; Editing by Chizu Nomiyama)