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

Total, Wintershall, Repsol, Eni and BASF have said they are either slowing or stopping output in Libya. Austria's OMV said it might be heading for a full production shutdown

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 two percent of world output. About 1.3m bpd is exported, mainly to Europe.

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.

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.

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