Energy giant Royal Dutch Shell said on Tuesday that about two barrels of oil a day are still spilling into the North Sea from its Gannet Alpha platform as it struggled to stem a second leak.
"The primary leak in the flow line is pretty much dead," said Glen Cayley, technical director of the Anglo-Dutch oil company's exploration and production activities in Europe, six days after the leak began.
"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."
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 about two barrels.
There was no question of it hitting the shore, he 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.
The amount spilled is bigger than the total amount spilled in any year in British waters over the last 10 years, according to figures from the Department of Energy and Climate Change (DECC).
"Although small in comparison to the Macondo-Gulf of Mexico incident, in the context of the UK continental shelf the spill is substantial," a DECC spokesman said on Monday, adding that it was "disappointing".
The oil spilled 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.
Explore further: North Sea oil leak biggest in decade: Britain