An oil spill off the south coast is thinning out as it spreads eastwards towards Britain, the Department of Transport has said.

The department halved an earlier estimate of the size of the spill to 522 tonnes and said it was naturally reducing as it moves east.

It is currently 63 km off Ireland's south-western coast.

'It's possible some of it may turn up on our southeast coast,' Irish Coast Guard Director Chris Reynolds told RTÉ Radio.

'If the oil stays on the surface as expected, the Welsh coast will get quite a bit of it as well. A lot depends on the weather,' Mr Reynolds said.

Earlier, the Coast Guard said the slick could land on the Wexford coast within 16 days.

The Russian Navy has admitted that two of its vessels refuelled near the site of the incident, but denied they were responsible.

The Department of Transport said aerial surveillance on Saturday confirmed the spill surrounding a Russian aircraft carrier and a refuelling tanker.

A navy spokesman told Reuters this morning there had not been an accident onboard, nor deliberate dumping of fuel overboard.

Igor Dygalo disputed the size of the oil spill, saying it neither 'has a catastrophic character nor constitutes a threat to coastal ecology'.

While some of the oil will evaporate or dissolve, a lot of oil remains on the water.

The Coast Guard was to send a tug from Cork in a bid to assess if it is possible to recover some of the oil on the water. A tanker is available in Cork to carry out the work if required.

However, the spokesman said the international experience of such work 'was not high', particularly after the oil has been on the water for some days and spread.

Spraying the oil, in order to break-up the slick, needed to happen within 12 hours to have any chances of success.

The Coast Guard is monitoring the situation, and a statement from the Department of Transport and Marine says further aerial pollution surveillance flights will be carried out.

According to the Coast Guard, vessels currently on the scene include two refuelling tankers, one Russian aircraft carrier, one Russian tug, one Russian destroyer, one British destroyer and the LÉ Aisling.