PARIS — World oil supplies reached their highest level in a decade in the fourth quarter as OPEC oil production edged up slightly in December because of higher Iranian output, the International Energy Agency said today.

Oil supplies in the non-communist world in the fourth quarter of 1989 averaged 54.8 million barrels a day, 1.5 million barrels more than during the same period in 1988, said the IEA, the agency maintained by the Western industrialized nations to monitor world energy.

The IEA said production of the 13-nation Organization of Petroleum Exporting Countries rose 100,000 barrels a day over November to a daily output of 23.9 million barrels,

The IEA said that during the fourth quarter, OPEC daily crude oil production averaged 23.7 million barrels, the highest figure since the first quarter of 1981.