By Terray Sylvester

CANNON BALL, N.D. (Reuters) - Tribal leaders protesting the construction of a controversial North Dakota pipeline vowed on Tuesday to fight U.S. President Donald Trump's order to revive the $3.8 billion project, calling his decision a "bad move."

Protesters have rallied for months against plans to route the Dakota Access pipeline under a lake near the Standing Rock Sioux reservation, saying it threatened water resources and sacred Native American sites.

The tribe, which has fought to stop the pipeline since last year, won a major victory last month when the government denied Energy Transfer Partners LP the right to run the pipeline under Lake Oahe, a water source upstream from the reservation.

Trump's order instructed the Army and the Army Corps of Engineers to review the decision.

The Republican president also signed an order reviving the C$8 billion ($6.1 billion) Keystone XL pipeline project, which was rejected in 2015 by then-President Barack Obama.

As a small airplane circled over the main protest camp near the Dakota Access pipeline on Tuesday, the mood following the White House's announcement was calm but defiant.

“I’m staying here,” Benjamin Buffalo, a 45-year-old Blackfeet tribal member from Browning, Montana, told a reporter. “I’m standing with the natives. This is our future.”

Buffalo has been at the camp since August, when tensions started to flare up between law enforcement officers and protesters, who have been backed by Hollywood celebrities, veterans and other activists.

The tribe had recently called for protesters to leave after the Army Corps of Engineers agreed to an environmental review last month, saying the battle had moved beyond the camp and into the courts or back rooms for negotiations with the government.

The tribe also warned that the camp itself might contaminate the river if hit by heavy flooding in March, when waters are expected to rise.

On Tuesday, Standing Rock leaders said they would meet in the coming days to plan next steps. Some said they feared fresh violence after past clashes between protesters and law enforcement officers.

Dana Yellow Fat, Standing Rock Sioux tribal council member at large, called Trump's order "a poor decision and a bad move" and said he worried about injuries if new violence broke out.

“Now you’re going to see both sides gear up for even more actions on the ground because you have a group of people that is determined to stop that pipeline one way or another,” he told Reuters.

Yellow Fat said he was unsure whether the tribe would back away from its request for protesters to leave the camp, but said Trump's order has prompted "a total re-evaluation of our recent actions."

Since the exit of the Standing Rock Sioux, the camp has been less organized, with no regular sunrise prayers and communal kitchens that now only serve food sporadically. In some spots, tents are buried under snow and as many as 60 cars have been abandoned.

Tribal officials expect the cleanup of the site to take about a month.

The Morton County Sheriff's Department urged activists to remain peaceful in light of Trump's order and said they were bracing for a possible resurgence in protests.

“We’re preparing for anything that might come,” department spokeswoman Maxine Herr said. “We continue to monitor the situation.”

She declined to say whether additional officers would be sent to the protest site.

Morton County spokesman Rob Keller on Monday said police had no plans to forcibly remove people from the campsite, where protesters now number 500 to 600, down from the nearly 10,000 once there.

Many in the camp, some of them members of Native American tribes from other parts of the country, had already planned to defy the Standing Rock Sioux Tribe's call to leave, saying the fight against the pipeline was not over.

Forest Borie, 33, of Magalia, California, said the protest will only become more intense.

"Our struggle to protect the planet is getting more intense, and the stakes are getting higher, said Borie, who has been at the camp since early November.

(Reporting by Terray Sylvester; additional reporting by Timothy Mclaughlin in Chicago; writing by Ben Klayman; editing by Jonathan Oatis)