Construction of the Dakota Access pipeline under a North Dakota reservoir has begun and it should be operational within three months, the developer of the long-delayed project said.
It comes as an American Indian tribe filed a legal challenge to block the work and protect its water supply.
The army granted Energy Transfer Partners (ETP) formal permission on Wednesday to lay pipe under Lake Oahe, clearing the way for completion of the 1,200-mile, 3.8 billion dollar (£3 billion) pipeline.
ETP spokeswoman Vicki Granado confirmed early on Thursday that construction resumed "immediately after receiving the easement".
Workers had already drilled entry and exit holes for the crossing, and oil had been put in the pipeline leading up to the lake in anticipation of finishing the project.
"The estimate is 60 days to complete the drill and another 23 days to fill the line to Patoka," Ms Granado said, referring to the shipping point in Illinois that is the pipeline's destination.
Work was stalled for months due to opposition by the Standing Rock Sioux and Cheyenne River Sioux, as well as a prolonged court battle between the developer and the Army Corps of Engineers that oversees the federal land where the last segment of the pipeline is now being laid.
President Donald Trump last month instructed the Corps to advance pipeline construction.
The Cheyenne River Sioux on Thursday asked a federal judge to stop the Lake Oahe work while a lawsuit filed earlier by the two tribes against the pipeline proceeds.
Lawyer Nicole Ducheneaux said in court documents that the pipeline "will desecrate the waters upon which Cheyenne River Sioux tribal members rely".
ETP did not immediately respond in court to the filing and U S District Judge James Boasberg did not immediately rule.
Standing Rock Sioux lawyer Jan Hasselman has said the tribe will also try to block the lake crossing in court.
An encampment near the construction site has drawn thousands of protesters since April in support of the Standing Rock Sioux, leading on occasions to clashes with law enforcement and hundreds of arrests.
Ms Granado said she was not aware of any incidents involving pipeline opponents in the area.
The Morton County Sheriff's Office also said it had not responded to any incidents.
Tribal chairman Dave Archambault said in a statement late on Wednesday that the tribe is prepared to keep up the battle "in the courts".
"We will continue to fight against an administration that seeks to dismiss not only our treaty rights and status as sovereign nations, but the safe drinking water of millions of Americans," the chairman said.
The tribe fears a leak in the pipeline could contaminate drinking water at its reservation that is just downstream from the proposed Missouri River crossing.
ETP said the pipeline will be safe.
In a statement on Wednesday, North Dakota governor Doug Burgum urged "co-operation and restraint" from all parties and requested federal law enforcement assistance to keep the peace during construction that would transport North Dakota oil across four states to a shipping point in Illinois.
Protesters rallied in several cities across the country on Wednesday.
Demonstrators in Chicago targeted a bank. Others went to an Army Corps of Engineers office in New York City but were asked to leave when they started filming without a permit.
Several people were arrested for blocking public access to a federal building in San Francisco.
Joye Braun and Payu Harris, two pipeline opponents who have been at the North Dakota encampment that has been the focus of the pipeline battle since April, said in an interview at a nearby casino that there is frustration but also resolve in the wake of the army's decision.
"The goal is still prayerful, non-violent direct action," Braun said.
The tribe maintains the decision violates its treaty rights, and its lawyers have vowed to keep fighting in court.
The Corps has notified protesters still at the encampment that the government-owned land will be closed on February 22.
But according to Mr Harris, a new camp is being established on private land.
"This is not over. We are here to stay. And there's more of us coming," he said.
An assessment conducted last year determined the river crossing would not have a significant effect on the environment.
However, the army in December decided further study was warranted to address tribal concerns.
The Corps launched an environmental impact study on January 18, but Mr Trump signed an executive action six days later telling the Corps to allow the company to proceed with construction.