The Opec oil cartel's resolve to cut production to boost prices is fraying, says the International Energy Agency, the developed countries' watchdog.

The cartel, which produces more than a third of the world's oil, last month raised its output by 270,000 barrels a day, ending a seven-month run of ever-steeper cutbacks, the IEA said. The move is important because the rise in production together with weak global oil demand could increase inventories and depress prices.