The flow of oil from the worst spill in UK waters in the past decade, at one of Shell's North Sea platforms, has been "greatly reduced" but not yet stopped completely, the government said on Monday.

Conservationists warned that the leak could harm bird life in the area, at a delicate time in their development, as the oil company worked to minimise the damage.

Stuart Housden, director of RSPB Scotland, said: "We know oil of any amount, if in the wrong place, at the wrong time, can have a devastating impact on marine life. Currently thousands of young auks – razorbills, puffins and guillemots – are flightless and dispersing widely in the North Sea during late summer. So they could be at serious risk if contaminated by this spill."

Greenpeace criticised Shell for not being sufficiently open about the progress of the spill, which was first discovered on Wednesday but not announced publicly by the company until Friday.

Shell said on Monday that it could not quantify the volume of oil spilled so far, but put it at 1,300 barrels, with a residual leak of about 5 barrels a day that has still to be stopped. Last year's BP spill in the Gulf of Mexico was estimated to be spilling up to 70,000 barrels a day. Shell said it could not predict when the leak would be halted completely.

The oil on the sea surface is estimated at under half a square mile at present. "The high winds and waves over the weekend have led to a substantial reduction in the size of the oil sheen as can be seen from the current levels on the water," said Glen Cayley, technical director of Shell's exploration and production activities in Europe. "This is a significant spill in the context of annual amounts of oil spilled in the North Sea. We care about the environment and we regret that the spill happened. We have taken it very seriously and responded promptly to it."

Green campaigners said the incident raised questions over the safety of oil companies' plans to drill in deep water in the Arctic, as the North Sea is generally supposed to be the safest in the world in terms of spills. Ben Ayliffe of Greenpeace, which has been campaigning to stop further oil-drilling exploration in delicate environments such as the Arctic, said: "The North Sea is supposed to be ultra-safe – we are told spills can't happen there. Shell is looking to move into the Arctic where an oil spill would all but impossible to clean up. Events in the North Sea should give the company pause for thought."

However, government advisers took the view that the risk to bird life would be small as the oil is unlikely to reach shore. The Department of Energy and Climate Change said: "Shell has informed us that the oil leak at its pipeline at the Gannet Alpha platform east of Aberdeen is under control and has now been greatly reduced. They are working to completely halt any further leakage. Although small in comparison to the Macondo incident [at one of BP's platforms in the Gulf of Mexico last year], in the context of the UK's continental shelf the spill is substantial – but it is not anticipated that oil will reach the shore and it is expected that it will be dispersed naturally."

Shell is using remotely operated vehicles to stop the remaining leak and to monitor the effects. A standby vessel, "Grampian Prince", is also monitoring the area, and is equipped with oil spill response equipment and dispersant, should that be required. The Maritime and Coastguard Agency are making twice daily flights to monitor the spill.