* OPEC keeps existing supply curbs in place
* Calls another meeting on May 28
* Focus on weak economy, G20 summit in April
* U.S. welcomes decision
(Updates with U.S. reaction, paragraph 3)
By Simon Webb and Alex Lawler
VIENNA, March 15 (Reuters) - OPEC ministers agreed on Sunday to leave existing output targets unchanged, but promised to enforce those curbs more strictly and said they would meet again at the end of May to review progress.
The decision to resist additional cuts for now reflected concern for the economy and a belief supply curbs so far have begun to remove the excess from oil markets, the Organization of the Petroleum Exporting Countries said in a communique after its nearly five-hour conference.
U.S. Energy Secretary Steven Chu said he was pleased with OPEC's decision, although he still believed the United States should seek to become energy independent.
Ministers from the 12-member producers' club had repeatedly said their focus was better compliance with deals in place since September to lower targets by 4.2 million barrels per day (bpd).
OPEC adherence has been estimated at roughly 80 percent and full compliance would take away more than 800,000 bpd more.
"It makes no sense to propose a cut if previous agreements have not been fulfilled," Venezuelan Minister of Energy and Petroleum Rafael Ramirez told reporters.
Venezuela in the past has been among the first to call for aggressive action to shore up prices.
But ahead of Sunday's meeting only Algeria had clearly spoken out in favour of another cut, saying the oil market had factored in a reduction of at least 500,000 bpd and prices would fall without further supply restraints.
For consumer nations, cheaper oil equates to a huge financial stimulus.
Barack Obama, president of the world's biggest energy consumer the United States, called Saudi King Abdullah last week.
The White House did not disclose the contents of the call, but analysts said the timing was significant.
They predicted leading OPEC producer Saudi Arabia would not want to be seen to be destablising the economy ahead of a Group of 20 summit in London in April, to which the kingdom is invited to help seek solutions to the world's financial crisis.
Asked about the new regime in the United States, OPEC's Secretary General Abdullah al-Badri said it marked an improvement in relations.
"I don't want to say that I voted for Obama, but we can see a different tone ... that we didn't see in the past. We have seen a positive approach. They are ready for dialogue and we are ready for dialogue and ready for talk," he said.
DONE ENOUGH?
The International Energy Agency, which advises consumer countries, in a report on Friday said the OPEC supply curbs already in place were enough to drain fuel stocks, even though it expected 2009 oil demand to fall by more than a million bpd compared with last year.
"I think generally speaking it's a sensible decision and one that the IEA could welcome given the strains that are being faced by the global economy," David Fyfe, head of the oil industry and markets division at the IEA, told Reuters.
"The last thing we need in the short term is an abrupt surge in oil prices."
OPEC's cuts since last September have helped to pull prices up from a low of $32.40 in December to around $46 now.
But levels are still just over $100 below last year's record high of nearly $150 and the group cannot forget the price crash of the late 1990s when oil fell towards $10 a barrel.
Although cheaper oil in the short term can help offset what the OPEC statement referred to as "the worst global economic recession in decades", OPEC ministers have said the risk is that it inhibits investment in new production, which will drive the price back up again.
Saudi Arabian Oil Minister Ali al-Naimi echoed comments by King Abdullah last year that a reasonable price was about $75.
"It's what we have said before. If you want the marginal producers to produce, all the poor guys that are shutting down their wells now, they need something about $70-$75. Everybody would be happy," Naimi said.
The other big risk as far as OPEC is concerned is that inventories will rise sharply as demand shrinks in line with a seasonal fall in energy use as well as the downturn in economic activity.
Oil stocks already equate to nearly 59 days of forward cover -- compared with the 52 days OPEC considers comfortable.
As a precaution, the producer group has called another meeting on May 28 at its headquarters in Vienna.
Analysts predicted the immediate price reaction when the oil market resumes trade on Monday would be negative, but that was no bad thing, even for OPEC, they said.
"At the moment, getting the world back on its feet is more important than lifting the oil price by a further $10 a barrel," said Lawrence Eagles of JP Morgan in New York.
"The world economy is crucial. Short-term gain would be to the long-term detriment of OPEC."
For related graphic, showing the relationship between OPEC output cuts and the oil price, please click on:
https://customers.reuters.com/d/graphics/OL_OPEV0309.gif (For a factbox on past OPEC production changes)
(Additional reporting by David Sheppard, Henrique Almeida and Karin Strohecker, Writing by Barbara Lewis; editing by William Hardy)