In the wake of the news on Thursday, world oil prices rebounded slightly in London and New York, traders said.
"At 9:00 pm (1700 GMT), a device exploded under a bridge, which collapsed. Eight parallel pipelines, feeding the Zubeir 1 and Zubeir 2 oilfields were damaged," said a source at the South Oil Company on condition of anonymity.
Threats against oil infrastructure amid fighting between US occupation troops and al-Mahdi Army fighters in Najaf, saw world oil prices reach record highs before southern Iraqi exports resumed to normal levels last weekend.
Out of a cluster of 20 pipelines at al-Barjassiya, eight were damaged in the explosion, said an official at the oil ministry in Baghdad.
"The fire still had not been put out on Thursday morning. This sabotage will affect exports," he said, refusing to elaborate.
Oil prices have closely reflected
events in the Middle East
The oilfields in question lie 20km southwest of Iraq's main southern city of Basra, said the South Oil Company.
The price of benchmark Brent North Sea crude oil for delivery in October climbed 22 cents to $40.90 a barrel in opening deals, having shed $1.64 the day before.
New York's main contract, light sweet crude for October delivery, gained 31 cents to $44.78 a barrel in pre-opening electronic trading, having plunged $1.74 on Wednesday.
Southern oil exports had returned to their normal level of 85,000 barrels an hour, or about 1.9 million barrels a day, at the weekend.
For the previous 13 days, exports had sunk to between 36,000 and 42,000 barrels a day due to threats by fighters loyal to Shia leader Muqtada al-Sadr to blow up oil pipelines feeding the two southern terminals.
In normal times, Iraq exports about 1.8 million barrels a day through its southern terminals.
The disruption to Iraqi exports helped world prices to record highs of approaching $50 in New York last week.