Opec has forecast that world demand for its crude will decline next year as rivals pump more, pointing to a market surplus in 2018 despite an Opec-led output cut, a Reuters report said.

Giving its first 2018 forecasts in a monthly report, the Organization of the Petroleum Exporting Countries said the world will need 32.20 million barrels per day (bpd) of crude from its members next year, down 60,000 bpd from this year, the report said.

Meanwhile, Opec said its oil output in June rose above the demand forecast, led by gains in Libya and Nigeria, two members exempt from the cut aimed at eliminating excess supply. Opec officials nonetheless remain upbeat on the outlook.

"We remain very optimistic ... (about) helping the market to rebalance itself," Opec Secretary-General Mohammad Barkindo said at an industry conference in Istanbul.

Oil rose above $48 a barrel on Wednesday as a US report of falling inventories in the US raised hopes that the glut is easing.