Opec's 10 members subject to output limits, all except Iraq and Angola, are set to pump 26.9 million barrels per day, up 100,000bpd from June, said Conrad Gerber, head of Petrologistics, which tracks tanker shipments.

The estimate, while showing rising supply in some Opec countries, indicates top exporter Saudi Arabia is keeping a cap on output in spite of a jump in oil prices towards a record high and consumer calls for more production.

"There's no major opening of the taps," Gerber said. "They fear that if they opened the taps, prices would slide."

Nigeria is raising supply in July by about 100,000bpd to 2.12 million bpd, Gerber said. The increase reflects fewer disruptions to the country's oil industry from militant attacks in the Niger Delta.

Iranian oil output is also on the increase — climbing by 50,000bpd to 3.95 million bpd.

Overall supply from the 12-member Organisation of the Petroleum Exporting Countries is set to rise 300,000bpd to 30.7 million bpd, Petrologistics said, as Iraq and Angola pump more.

Iraqi output is on course to reach 2.08 million bpd, up from 1.94 million bpd in June, because the country is exporting some Kirkuk crude from its northern fields.

Storage tanks at the Turkish port of Ceyhan receive sporadic deliveries of Kirkuk by pipeline from Iraq's northern oilfields. Iraq sold three million barrels for shipment in July, the first such sale since January.