LONDON — The Organization of the Petroleum Exporting Countries issued a report on Thursday that downgraded demand for its crude for 2015, while also predicting slower oil-production growth in the United States.

The report, which was prepared by the cartel’s economists at its headquarters in Vienna, said that demand for OPEC crude would be 28.8 million barrels a day this year, 100,000 barrels a day less than it forecast in its previous monthly market report, in December.

The report also said that demand for OPEC’s crude would average less than 28 million barrels a day in the first half of this year.

OPEC, a group of 12 oil-producing nations with Saudi Arabia as the linchpin, showed no sign of cutting production from the average of about 30 million barrels a day in 2014. Its forecasts imply an oversupply of more than 2 million barrels a day in the first half of this year.