Oil's two-year bull run has robbed the Organisation of the Petroleum Exporting Countries of its power to control prices that have roared above $70.

Pledges by the cartel and its lead producer Saudi Arabia to pump more and more oil to make up any shortages from Hurricane Katrina's rampage through the US Gulf rang hollow with those awash in crude yet starved of petrol and other oil products.

"Opec is no longer in the driver's seat," said oil consultant Geoff Pyne. "Oil prices are not a simple matter of crude supply and demand."

The 11-member producer group, already pumping at 26 year highs to cover booming Asian and US oil demand, has struggled to tame a rally that has more than doubled prices since 2003.

Opec officials blame much of that run-up on the world's shortage of sophisticated refineries to process their additional high-sulphur, sour crude supplies.

But as oil hit a record $70.80 last week, Opec and Saudi Arabia, the only producer in the world with significant excess production capacity, stepped up to offer still more crude.

A "very worried" Opec president Sheikh Ahmad al-Fahd al-Sabah, the Oil Minister of Kuwait, said he would propose a 500,000 barrels per day (bpd) rise in Opec supplies when the group meets next month.

He said most of the extra would come from Saudi Arabia. Riyadh later vowed to boost output by 1.5 million bpd and pump full blast at 11 million if necessary to help cover shortages from Katrina.

Some oil industry executives said the Saudi promise, however well-meaning, could even have the opposite effect from that intended as it could leave Opec with little or no spare capacity to cope with emergencies.

"The price of oil is at $70 because of concerns about supply security and spare capacity," said an executive of a major buyer of Saudi oil. "Why would those concerns disappear with Opec eroding most of its spare capacity and US output down?"

US output was cut by about 1.4 million bpd as production was shut in ahead of the hurricane to try to prevent damage.

That roughly equates to the spare capacity held by Opec.

Adding to market fear is the fact that some oil industry executives are unconvinced of Saudi Arabia's ability to pump 11 million bpd for long periods.

"The Saudi's 11 million is a symbolic number," said Mehdi Varzi, president of consultancy Varzi Energy.

"This is an attempt to prevent oil prices from going higher, but I'm not sure what Saudi Arabia and Opec can do."

Mr Varzi and other oil experts reckon Riyadh can sustain crude production of about 10.5 million bpd. With the kingdom now pumping about 9.5 million, that leaves about one million bpd of mostly heavy crude to spare, they said. That hard-to-refine sour crude is not what oil companies want. Other oil dealers said they were reassured that the kingdom still had the ability to open up its taps.

"The point is that Saudi Arabia is saying it still has some capacity spare," said Mark Keenan, a fund manager at MPC Commodities Fund.

"It goes some way to addressing the value of crude oil." It does not, however, solve the problem for refiners in the US Gulf, many of whom lack the high-tech units required to process medium and heavy Saudi crude into transport fuels.

"Putting more sour crude on the market won't solve the crisis," said a Gulf oil executive.

Apart from the promise of extra pec oil, the United States said it would dip into its strategic reserves if necessary.

The US Department of Energy loaned out 5.4 million barrels after Hurricane Ivan disrupted supplies last year, but this was a rare measure.

"The SPR is not the answer because they aren't going to release the oil, they are going to loan it. That means you have to replace it later," a Western oil executive said.