"There is no doubt that oil was discharged into the Hudson River," Cuomo told reporters Sunday, speaking at the plant. "Exactly how much, we don't know."
The New York State Department of Environmental Conservation was on scene in Buchanan, New York, at the site of the fire and on the river Saturday night into Sunday, monitoring what Commissioner Joseph Martens called an oil sheen of up to 300 feet in diameter.
The transformer that failed ruptured during the fire, leaking oil onto the ground, Cuomo said. The oil was first caught in a holding tank but spilled over into a Hudson-bound drainage system after reaching capacity.
A spokesman for the federal Nuclear Regulatory Commission estimated the volume of oil spilled to be several thousand gallons, though Cuomo and Martens would not confirm the number.
Martens said the slick is contained by booms in the water and that an absorbent material will be released into the river in the coming days to soak up the oil for its removal.
"We were here last night into the wee hours and back again this morning and we will be onsite working with the cleanup team to make sure that any damage is minimized to the Hudson River," he said.
The fire broke out just before 6:00 p.m. on the non-nuclear side of the plant, about 200 yards away from the reactor building, according to Entergy spokesman Jerry Nappi.
One nuclear reactor unit automatically shut down during the blaze, according to Nappi.
In a tweet, the facility said that there was "no threat to public safety at any time" and that "all Indian Point emergency systems worked as designed."
No one was injured in the blaze, though responding to a question about the impact on wildlife in the river, Cuomo told reporters on Sunday "it's obviously not good."
The exact damage to the ecosystem will be determined after further investigation, he added.
"We saw just a huge black ball of smoke right across the river," witness Gustavus Gricius told CNN. "We could smell the oily, electric burn smell."
The blast sent the facility, located approximately 35 miles north of midtown Manhattan, into an emergency response situation classified as an "unusual event," according to Nappi.
Neil Sheehan, a spokesman for the federal Nuclear Regulatory Commission, said the agency had three inspectors respond.
"They're cooling down the reactor, and we'll have to investigate the cause of the fire," he said on Saturday.
The facility houses two nuclear reactor units and produces approximately 25% of the electricity for New York City and Westchester County, according to its website.