VIENNA (Reuters) - OPEC ministers on Wednesday announced a surprise output cut following a fall in oil prices to just above $100 a barrel.

Oil rose by a dollar immediately after the announcement.

OPEC President Chakib Khelil said the group would in effect be cutting roughly half a million barrels per day (bpd) from the group's July output level over the next 40 days.

"I think if you do your own calculation properly, it will be a lowering of production by about 520,000 barrels per day," Khelil said.

After nearly five hours of debate, the Organization of the Petroleum Exporting Countries (OPEC) said it had returned to a production ceiling of 28.8 million barrels per day (bpd), previously in effect a year ago.

The meeting, which did not begin until late on Tuesday because of Ramadan fasting, had been expected to keep output targets unchanged.

But on Tuesday oil prices fell to a five-month low below $102 a barrel, about 30 percent below the all-time high of $147.27 hit on July 11.

OPEC's previous production target was 29.67 million bpd for the 12 members with output limits.

According to secondary sources, OPEC is estimated to be producing roughly 790,000 barrels per day bpd above that ceiling