As much as a quarter of OPEC member Libya's oil output has been shut down, Reuters calculations showed on Wednesday, as unrest prompted oil firms to warn of production cuts in Africa's third-largest producer.

Brent crude prices topped $110 a barrel for the first time since 2008 because of the disruption, even though Saudi Arabia has said other members of the Organization of the Petroleum Exporting Countries would be ready to meet any shortages.

Austria's OMV said on Wednesday it might be heading for a full production shutdown in Libya. Total, Repsol, Eni and BASF

have also said they are either slowing or stopping output.

The latest comments point to a growing impact on oil output from Libya, which produces 1.6 million barrels per day (bpd) of high-quality oil, or almost 2 percent of world output. About 1.3 million bpd is exported, mainly to Europe.

We are evaluating the situation. We cannot say at the moment how production is developing exactly, OMV Chief Executive Wolfgang Ruttenstorfer told a news conference.

It is going down sharply. We do not rule out that it could come to a complete stop for a period of time.

The figures given by oil companies and industry sources so far indicate that 300,000-400,000 bpd of Libyan output -- up to a quarter of the total -- has been stopped, according to Reuters calculations.

That total could well be higher since it excludes Eni, which said on Tuesday it suspended some Libyan output without giving details. Libya provided Eni with oil production of 108,000 bpd in 2009, the latest data available.

Information can be conflicting on the country's output because oil companies often speak of their share of production and do not give overall supply at fields they operate or participate in.

We have started to suspend our production. It is still too early to estimate the impact on our production, a Total spokeswoman said on Wednesday. Total gets 55,000 bpd from Libya.

OMV SHARES HIT

OMV shares tumbled 5 percent on the news in Libya, which provided the company with 33,000 barrels of oil equivalent per day of output in 2010, around a tenth of its total output.

Spain's Repsol and Italy's Eni said on Tuesday they had shut down Libyan production. BASF unit Wintershall said on Wednesday it had stopped as much as 100,000 bpd of output, having said on Monday it was preparing to do so.

Repsol said it had shut the El-Sharara oilfield, which an industry source said pumps about 200,000 bpd -- a figure equal to 13 percent of the country's output.

Libyan export terminals that ship both crude and oil products remain disrupted, industry sources said.

Libyan oil officials could not be reached by phone to provide information on output and exports.