Near Cannon Ball, North Dakota (CNN) Celebrations, tears of joy, chanting and drumming rang out among thousands of protesters at the Standing Rock site after the Army Corp of Engineers announced it will look for an alternate route for the Dakota Access Pipeline to cross under Lake Oahe in North Dakota.

For months, members of the Sioux tribe and their supporters have camped out, fighting the pipeline they say could be hazardous and damage the water supply of their reservation nearby.

"People have said that this is a make it or a break it, and I guess we made it," Chief Arvol Looking Horse, Keeper of the White Buffalo Calf Pipe, announced to a cheering crowd of protesters.

Tribal leaders worry the decision to change direction may not be permanent, especially with the incoming Trump administration, as well backers of the pipeline seeking to push the project ahead.

Grassroots activists, who have turned the protest site into a mini-city, prepared to withstand freezing temperatures during what was expected to be an even lengthier standoff, were cautious about the scope and durability of their victory.

"I'm really happy that I'm here to witness it and celebrate with a lot of my elders and the youth, but I think that we also need to keep in mind that we need to be ready to keep going," said protester Morning Star Angeline Chippewa-Freeland.

"We are asking our supporters to keep up the pressure, because while President Obama has granted us a victory today, that victory isn't guaranteed in the next administration," Dallas Goldtooth, lead organizer for the Indigenous Environmental Network, said in a statement. "More threats are likely in the year to come, and we cannot stop until this pipeline is completely and utterly defeated, and our water and climate are safe."

Why reroute the pipeline?

The Army Corps of Engineers said it will not grant a permit to allow the proposed pipeline to cross under the lake. Officials said after discussion with the tribe and Dakota Access it became clear that more work had to be done on the project.

"The best way to complete that work responsibly and expeditiously is to explore alternate routes for the pipeline crossing," The corps' assistant secretary for civil works, Jo-Ellen Darcy, said in a statement.

The decision comes three weeks after her office announced it was delaying the decision after protests from the Standing Rock Sioux tribe and its supporters.

Darcy said the consideration of alternative routes would be best accomplished through an environmental impact statement with full public input and analysis, delivering both an immediate reprieve and political statement that could aid in future showdowns with President-elect Donald Trump's incoming administration.

Pipeline supporters speak out

Energy Transfer Partners and Sunoco Logistics Partners, the corporations behind the Dakota Access Pipeline, said in a statement Sunday night they "fully expect to complete construction of the pipeline without any additional rerouting in and around Lake Oahe."

As far as they're concerned, the White House's directive does not change past court decisions to green light the project.

"Nothing this Administration has done today changes that in any way," the company said in a statement.

North Dakota Sen. John Hoeven, a Republican, said last week after a meeting with the transition team that Trump supported completing the 1,172-mile long proposed pipeline, that would transport 470,000 barrels of oil a day across four states. A spokeswoman for Trump did not immediately respond to a request for comment on Sunday's decision.

House Speaker Paul Ryan tweeted his criticism, calling the intervention "big-government decision-making at its worst. I look forward to putting this anti-energy presidency behind us."

This is big-government decision-making at its worst. I look forward to putting this anti-energy presidency behind us. https://t.co/Qu0nFTmGZv — Paul Ryan (@SpeakerRyan) December 5, 2016

North Dakota's sole member in the House of Representatives, Rep. Kevin Cramer, a Republican, pledged to fight on and slammed President Obama.

"I hoped even a lawless President wouldn't continue to ignore the rule of law. However, it was becoming increasingly clear he was punting this issue down the road," Cramer wrote in a statement. "Today's unfortunate decision sends a very chilling signal to others who want to build infrastructure in this country."

Opponents ready for next fight

The decision by the Corps of Engineers would be useful in a court challenge, according to Jan Hasselman, an Earthjustice staff attorney representing the Standing Rock Sioux tribe.

"If the incoming administration tries to undo this and jam the pipeline through despite the need for an analysis of alternatives, we will certainly be prepared to challenge that in court," he said. "It's not so simple for one government administration to simply reverse the decisions of the former one."

Sen. Bernie Sanders, who actively opposed the pipeline, praised the administration's decision.

"I appreciate very much President Obama listening to the Native American people and millions of others who believe this pipeline should not be built," Sanders, a 2016 Democratic presidential candidate, said in a statement. "In the year 2016, we should not continue to trample on Native American sovereignty. We should not endanger the water supply of millions of people."

May Boeve, the executive director leading environmental action group 350.org, celebrated the decision but also sounded a warning against any future plans to reverse it.

"If Trump tries to go up against the leaders at Standing Rock he'll just end up looking petty and small," she said. "The fight against Dakota Access has fired up a resistance movement that is ready to take on any fossil fuel project the Trump administration tries to approve. On Dakota Access and every other pipeline: If he tries to build it, we will come."

From threat of removal to celebration

Earlier this week, North Dakota Gov. Jack Dalrymple ordered the protesters to leave the campsite by Monday, citing the harsh weather conditions as a reason why they needed to decamp. The US Army Corps of Engineers had warned that come Monday, activists who refused to leave the campsite could be arrested, then backtracked, saying the agency had no plans to forcibly remove those who stay.