LONDON (AP) – Irish authorities said Tuesday they are monitoring a major oil spill that is drifting toward the Irish coast – the largest spill in the waters around Ireland in a decade.

The Irish Marine Department said the oil slick was discovered close to where a Russian aircraft carrier was refueling in the Celtic Sea between western Britain and the southern coast of Ireland.

The department said Tuesday it was too early to predict how much of the spill, thought to be around 500 tons (3,750 barrels), will come ashore. The oil slick is about three miles long and three miles wide.

Molly Walsh, a spokeswoman for the environmental group Friends of the Earth, said the spill could seriously damage marine life.

Irish authorities learned about the spill Saturday through surveillance carried out by the European Maritime Safety Agency in Lisbon, Portugal. Irish military aircraft flew over the area and saw the Russian aircraft carrier Admiral Kuznetsov, a Russian oil tanker, and a Russian oceangoing tug near the slick.

Russia’s chief of general staff Gen. Nikolai Marakov confirmed that a Russian aircraft carrier had refueled in the area but denied there had been any problems.

“We have no reason to think that anything went wrong during refueling,” he told reporters.

Ireland’s Department of Transport said it expects the slick to reach the southern coast of Ireland in about 16 days. It said some of the oil will evaporate and most of the rest will likely develop into tar balls – small, sticky patches of oil that often wash ashore.

The Press Association, the British news agency, said a Russian destroyer, a British destroyer, an Irish Naval vessel and a Russian aircraft carrier are at the site of the spill off the west coast of Ireland.

An Irish vessel set off Tuesday evening to assess whether the oil can be mechanically cleaned up at sea to limit the damage to coastline.

The Irish government has asked the Russian embassy in Dublin to hand over samples of the oils carried on board the Russian vehicles.

Earlier in the day, Russian Navy spokesman Capt. Igor Dygalo said there had been no problems with the Russian ships.

“There have been no accidents on Russian ships linked to broken pipes or burst fuel tanks, nor has there been any deliberate dumping of fuel into the sea,” he said in a statement carried by Russian news wires.

John Lucey, a biologist with Ireland’s Environmental Protection Agency said it was the biggest oil spill in the waters around Ireland in the last ten years

The last major spill in the area occurred in 1999, when the oil tanker Sea Empress ran aground in southwest Wales and spilled 72,000 tons of oil on the Welsh coastline.

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Associated Press Writer Vladimir Isachenkov in Moscow contributed to this report.

AP-ES-02-17-09 1729EST