World food prices slipped in September from the month before, with only sugar posting a rise, the United Nations food agency said on Thursday.

The Food and Agriculture Organization's (FAO) food price index, which measures monthly changes for a basket of cereals, oilseeds, dairy products, meat and sugar, averaged 165.4 points last month, against a marginally revised 167.7 in August.

The August figure was previously given as 167.6.

FAO said global cereals output in 2018 was seen at 2.591 billion tons, up three million tons on the previous forecast given in September, but still down 63 million tons, or 2.4 percent, from 2017's record production level.

FAO's forecast for world wheat production in 2018 was almost unchanged on 722.4 million tons, the smallest since 2013.

The U.N. body's Cereal Price Index averaged almost 164 points in September, down 2.8 percent from August.

Among the major cereals, maize export quotations registered the sharpest month-on-month decline down some 4 percent, mostly on expectations of a very large crop in the United States and good supply prospects globally.

Wheat price quotations also fell, mainly on continued strong sales and shipments from Russia.

By contrast, the FAO Sugar Price Index averaged 161.4 points last month, up 2.6 percent from August. FAO said the increase was largely linked to the ongoing harvesting in Brazil, the world's largest sugar producer and exporter.

"Drought conditions in Brazil during the critical growing season are seen to have had negative impacts on sugarcane yields, with harvested cane volumes falling below expectations," FAO said.