Shell said Tuesday that two barrels of oil per day were still spilling into the North Sea from its Gannet Alpha platform through a second leak, despite having brought most of it under control.

"The primary leak in the flow line is pretty much dead," said Glen Cayley, technical director of the Anglo-Dutch oil giant's exploration and production activities in Europe, six days after the leak was first identified.

"There is a small secondary leak created by that which is the small flow of two barrels a day, which is proving a little difficult to get to and isolate."

The oil leakage is equivalent to about 318 litres per day. One barrel of oil comprises just under 159 litres.

A Shell spokeswoman insisted this was not a new leak, saying that although the oil's initial path to the sea was stopped on Thursday, it had found a new outlet.

"We believe now that the flow is coming from a relief valve adjacent to the original leak and from the same source. Once we've confirmed this we will then develop a series of mitigation options to stop this leak," she said.

About 216 tonnes (1,300 barrels) of oil has leaked from the Gannet Alpha platform, 112 miles (180 kilometres) east of Aberdeen on the Scottish coast, Shell revealed on Monday -- making it the biggest in British waters in a decade.

Cayley said the oil on the sea surface had been "much reduced" due to harsh weather conditions, which had broken up a slick that at one point was 30 kilometres in length to two barrels.

There was no question of it hitting the shore, Cayley added.

"The leak that we've stemmed was in the flow line, so job number one was to close in the wells and isolate the reservoir, which of course is the large volume from the leak," Cayley said.

"We're confident that it's under control.

"The residual small leak is in an awkward position to get to. This is complex sub-sea infrastructure, and really getting into it amongst quite dense marine growth is proving a challenge."

He added: "Shell deeply regret this spill. We work very hard and invest heavily to ensure this doesn't happen and when it does we respond swiftly and decisively as we've done here."

He rejected criticism by environmental groups that the company took too long to make the spill public, saying that the authorities were informed as soon as Shell became aware of it last Wednesday.

Per Fischer, from campaign group Friends of the Earth Scotland, said: "It beggars belief that we are still being drip-fed information and that Shell's initially 'insignificant' leak is still causing problems."

The leak has spilled more oil than the total amount spilled in any year in British waters in the past 10 years, according to figures from the Department of Energy and Climate Change (DECC).

The leaking oil is light crude oil with a low wax content, and there is also some hydraulic fluid present, according to Shell.

The Gannet Alpha platform continues to operate.

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