By Ernest Scheyder and Valerie Volcovici

(Reuters) - The surprising victory by Native American and environmental groups in September to delay the Dakota Access Pipeline may turn out to be short-lived, after Donald Trump's unexpected win in the U.S. presidential election.

Trump backs measures to speed energy industry development and upgrade the country's oil and gas infrastructure. He has not commented specifically on the $3.7 billion Dakota Access line but has said he would seek to revive another controversial pipeline, the Keystone XL line. That project, which would pump oil from Canada through Nebraska, was rejected in 2014 by the Obama administration.

The 1,172-mile (1,885 km) line was planned to run from North Dakota's Bakken shale region to Illinois, but protests from environmental activists and the Standing Rock Sioux Tribe of North Dakota galvanized the Obama administration to delay construction to ensure Native American concerns about the line's route were properly addressed.

One day after Trump's victory, Standing Rock Sioux Chairman Dave Archambault II said the results show "that we as a country have so much work to do." He did not mention Trump in his statement to Reuters, instead saying President Barack Obama could still halt the pipeline.

"We must strengthen our resolve to protect the water, pray together for understanding, and pour our hearts and minds into the future of all our children," he said.

It is not clear now whether Dakota Access would be rerouted or piped under the sensitive watershed, which the tribe considers sacred.

Kelcy Warren, chief executive officer of Energy Transfer Partners, the company behind the pipeline, said a Trump presidency was a good thing.

"I view the results of last night's election as favorable not only for our project, but for future infrastructure projects that have been vetted and reviewed as thoroughly as ours has been," Warren said on Wednesday in a statement emailed to Reuters.

Representative Kevin Cramer of North Dakota, a Trump energy adviser, said he expects the Obama administration to approve the permit because the federal government is "out of legal reasons" not to allow the line to proceed, saying he did not think Trump "would have to weigh in with an opinion on this."

"We're especially hopeful that the pipeline approval process will be allowed to work without political interferences. We saw both Keystone XL and DAPL, which passed their NEPA review, get blocked for unrelated political reasons," said John Stoody, vice president of government and public relations for the Association of Oil Pipe Lines, referring to the National Environmental Policy Act review.

TRUMP IS "CLIMATE THREAT"

Trump carried the state of North Dakota by a wide margin, though his Democrat Hillary Clinton won Sioux County, which lies entirely within the Standing Rock reservation.

Environmental groups said they would continue to support the tribe, and looked to Obama to take any action he can to kill the pipeline.

"There is no doubt that Donald Trump poses an immediate threat to our climate and will try to fast track this and other fossil fuel projects across the country," Greenpeace spokeswoman Lilian Molina said in a statement.

"This is all the more reason for President Obama to step in immediately to stop the pipeline once and for all."

Trump's victory makes the completion of the pipeline more likely, analysts said. Energy Transfer Partners said Tuesday that it had completed construction to the edge of Lake Oahe, the location of the federal land where regulators delayed construction to review permitting.

Phillips 66, a 25 percent owner of the line, said 85 percent of the line is complete. Company spokesman Dennis Nuss said Wednesday that Phillips expects operations to begin in the first quarter of 2017.

Last week, Obama said that the U.S. government was looking for ways to move the line. Banks financing the line have been feeling the pressure from activists; Citigroup Inc on Tuesday said it had discussed concerns with Energy Transfer about reaching a resolution with the Standing Rock Sioux.

Trump has said he would ask TransCanada Corp, the principal owner of Keystone XL, to renew its application for the pipeline. TransCanada said Wednesday it is committed to building the project. "We are evaluating ways to engage the new administration on the benefits, the jobs and the tax revenues this project brings to the table," a spokesman said.

(Reporting By Ernie Scheyder and Valerie Volcovici; Additional reporting by Liz Hampton; Writing by David Gaffen; Editing by Andrew Hay and Leslie Adler)