HALIFAX -- Oil that spilled into Halifax harbour from a Nova Scotia Power generating station on Thursday was still being cleaned up a day later, but the utility had yet to disclose how big the spill was.

The utility would only say that the spill involved a "limited" amount of oil leaking into a small cove on the Dartmouth side of the busy harbour.

"The focus for today is to ensure that the oil is contained along the shoreline adjacent to the plant," the utility said in a statement Friday.

Tiffany Chase, a spokeswoman for the utility, said Friday afternoon that the company still didn't have an estimate of the amount of oil that spilled out of a small hole in a pipe onto the ground and from there into the harbour.

"The pipe itself that had a leak is five to seven inches in diameter and the hole is bigger than your thumb," she said during an interview.

"We weren't actually fuelling the plant with oil at the time ... So there was less pressure in the pipe at the time than if we had been streaming oil into the plant for fuel."

Crews aboard a helicopter and a boat organized the cleanup, which started soon after Nova Scotia Power employees discovered the leaking pipe during a routine inspection late Thursday.

The utility, a subsidiary of Halifax-based Emera Inc., said the exterior pipe runs from storage tanks to the Tufts Cove generating station.

"Plant staff immediately activated environmental response protocols by shutting off oil flow through the pipe to stop the leak," the utility said in a statement.

Environmental response crews set up three booms on the water and were using vacuum trucks and absorbent material to remove the oil.

The 500-megawatt plant largely uses natural gas to make electricity, but it can also run on oil.

Nova Scotia Power confirmed Friday it has launched an internal investigation. It also said more than 60 Nova Scotia Power employees and contractors were working on the cleanup.

"Nova Scotia Power and its contractor will continue to implement environmental response and monitoring protocols until all of the leaked oil has been cleaned up," the statement said.

"All appropriate operational procedures and protocols have been activated, and relevant regulatory bodies have been notified of the incident."

The company says the three oil-fired units at Tufts Cove were commissioned in the 1960s and 1970s, and were converted to also burn natural gas in 1999-2000. Its two natural gas-fired combustion turbines were commissioned in 2003 and 2004.