A foul-smelling black cloud of wastewater discharge appeared at the base of the Niagara Falls, initially prompting fears of an oil leak.

Seeping out near the dock where the popular Maid of the Mist boat tour of the renowned waterfalls operates from, it could be clearly seen from the cliff faces above the Niagara River, on both the Canadian and American side of the border.

Dismissing fears of an oil leak, officials said the colour caused by residue from black carbon filters which are used to clean the water.

It followed a maintenance operation at a local wastewater treatment plant, they said.

Pat Proctor, vice president for Rainbow Air Inc, which provides helicopter tours over the falls, was among the first to notice the black stain quickly colouring the water.

"I was just praying it wasn't an oil leak," he told the Buffalo News. "It looked like something out of a sci-fi movie."

In a statement, the Niagara Falls Water Board apologised for causing alarm.

It said: "The 'inky water' was the result of "a routine, necessary, and short term change in the waste water treatment process" at the plant, around four miles from the falls.

The board added that the water discharged contained some accumulated solids and carbon residue which were within permitted limits and did not include any organic type oils or solvents and that the smell would be limited to normal sewer water discharge.

New York Governor Andrew Cuomo nonetheless asked state regulators to investigate the wastewater discharges.