OPEC output has risen slightly in December as Nigerian supply increased, a Reuters survey found on Thursday, indicating the group has yet to boost production substantially in response to prices at a 26-month high.

Supply from the 11 members of the Organization of the Petroleum Exporting Countries with output targets, all except Iraq, has averaged 26.75 million barrels per day (bpd) this month, up from 26.70 million bpd in November, according to the survey of oil companies, OPEC officials and analysts.

The increase mainly reflects higher supply in Nigeria after a pipeline was fixed and other technical factors elsewhere in the group that affected output. OPEC officials have said a rise in prices to $100 a barrel -- above the 26-month high of $91.88 hit this week -- would not necessarily prompt a supply increase.

Until now production was adequate to meet demand, said Christophe Barret, oil analyst at Credit Agricole, who expects OPEC will need to boost supplies in 2011.

OPEC, source of more than a third of the world's oil, left its oil output target unchanged at a December 11 meeting, as it has since making a record supply curb of 4.2 million bpd in December 2008 as recession hit demand and prices.

Production has risen since then as prices and demand recovered. Even so, the survey found that the OPEC members with the most scope to boost supplies -- Saudi Arabia, Kuwait and the United Arab Emirates -- kept supply largely steady in December.

The market is well supplied and there is no shortage, said an OPEC delegate, who declined to be identified by name. Our only concern is the price, how high it will go.

The survey also found that the 11 members of OPEC bound by the December 2008 agreement to reduce their output delivered 55 percent of the reduction in December, compared with 56 percent in November.

Total OPEC supply including Iraq has risen 70,000 bpd, according to the survey.

OPEC quotas exclude condensate and natural gas liquids and apply to supply rather than wellhead output, defined to exclude movements to, but not sales from, storage. Saudi and Kuwaiti data includes Neutral Zone.

Saudi data excludes oil produced for Bahrain. Venezuelan data includes upgraded synthetic oil. Nigerian output includes the Agbami stream and excludes Oso and Akpo.

(Reporting by Alex Lawler; Editing by Barbara Lewis and Anthony Barker)