Irish aircraft monitoring an oil spill believed to come from a Russian navy refuelling accident off Ireland's south coast have recorded 522 tonnes of fuel spreading across the sea, officials said Tuesday.

Following an overfly of the spill, Irish authorities downgraded an earlier estimate of 1,000 tonnes given by the British coastguard, and said the fuel oil was now in three distinct slicks in the North Atlantic, heading east.

The spill, thought to have occurred at the weekend, was about 48 to 64 kilometres (30 to 40 miles) off the south coast and could eventually pollute the Irish or Welsh coastlines, the transport ministry said.

But it added it was too early to predict how much could come ashore, noting that oil had already begun dispersing late Tuesday.

Ireland has scrambled aircraft and a naval vessel to monitor the spill, and a tug was sent out to establish whether the oil can be mechanically recovered at sea, in theory starting on Wednesday.

Irish naval vessel Aisling and British destroyer HMS Gloucester have taken samples of the oil which will be sent for testing in Scotland.

Other vessels at the scene Tuesday included two refuelling tankers, one aircraft carrier, one Russian ocean-going tug and one Russian destroyer. The Coast Guard said it had established that no nuclear vessels are involved.

The Irish authorities were alerted on Saturday by a satellite surveillance pollution report from the European Maritime Safety Agency (EMSA) in Lisbon.

The ministry said aerial surveillance "confirmed an oil spill covering approximately 4.5 km by 5 km surrounding a Russian aircraft carrier and a refuelling tanker."

The Russian naval attache confirmed on Monday that the carrier had been carrying out a fuel transfer at sea -- called bunkering -- from a Russian supply tanker, the transport ministry said.

"The attache confirmed an internal investigation is being carried out into the cause of the incident and said that Russian aerial surveillance considered that approximately 300 tonnes of oil was on the sea surface," it said.

The statement said the attache "could not tell the Coast Guard how this happened or whether it was from their refuelling operations."

In Moscow, the head of the Russian chief of staff Nikolai Makarov confirmed that a group of Russian warships had been refuelling in the area but denied there had been any significant leaking of fuel.

"In this area a group of warships carried out refuelling but according to the commander of the group Admiral Korolev the refuelling proceeded normally and no significant outflow of fuel took place," he said, cited by Interfax news agency.

A Russian naval spokesman, Igor Dygalo, disputed the size of the oil spill, saying it neither "has a catastrophic character nor constitutes a threat to coastal ecology."

The Russian embassy in Dublin has been asked to supply samples of the oils carried on board the Russian tankers and the aircraft carrier and for oil characteristic data sheets.

"The oil spill is forecast to continue to disperse and to break up. The residual oil remaining is expected to develop into tar balls," the ministry said.

"Depending on weather conditions these may end up on the Irish south-east coast in approximately 16 days time and also impact on the Welsh coastline. At this point it is too early to predict accurate volumes."

burs/ar/boc