* IEA, OPEC revise downwards oil demand forecasts
* Inventory levels are high
* Global economic weakness overshadows talks
By Henrique Almeida and David Sheppard
VIENNA, March 13 (Reuters) - Swollen supplies and dwindling demand make a case for OPEC ministers to cut supply again at a weekend meeting, but the feebleness of the global economy and a firmer oil price complicate their task.
Both the Organization of the Petroleum Exporting Countries and the International Energy Agency revised downward their forecasts for global demand, reflecting the impact of lower economic output on energy use, in monthly reports released on Friday.
But the IEA, which represents consumer interests, said tighter compliance with curbs OPEC has already agreed would be enough to take the group's output a hefty 1.6 million barrels per day below 2009 demand for its oil.
OPEC's biggest and most influential member Saudi Arabia has yet to comment publicly.
Sources have said the kingdom, which has delivered the biggest share of cuts so far, has emphasised the need for tighter compliance from other members of the group.
Libya's most senior oil official agreed discipline was the first item on the agenda, but that alone might not be enough.
"Compliance is a must -- something else would be preferable," he told Reuters.
DEEP CUTS
Since last September, OPEC has already agreed output cuts totalling 4.2 million barrels per day (bpd). The cuts are the equivalent of about five percent of daily demand and are the deepest and most rapid yet.
So far compliance with those targets is historically high at around 80 percent, independent observers have said.
Inventory levels are also high and according to the IEA equate to 58.7 days of forward cover -- much more than the 52 days OPEC considers comfortable.
But oil prices have firmed. On Friday, U.S. crude traded around $47, well above a low of $32.40 hit in December, although $100 below last year's record high.
The IEA has warned prices could eventually race back up to record levels as reduced profit margins limit investment in new oil production. What the world needs at the moment is the fiscal stiumulus provided by relatively cheap oil, it said.
"We think that OPEC's existing targets will probably begin to tighten the market quite sharply from late in the second quarter onwards," said David Fyfe of the IEA's oil industry and markets division.
"And therefore OPEC cuts are already putting a floor under the price. What we probably don't need at the moment in this current short-term weak economic environment is a sudden surge in prices if OPEC were to decide to go even further on Sunday."
The IEA revised downwards its forecast for 2009 oil demand on Friday, saying it would shrink by 1.25 million bpd to 84.4 million bpd, while OPEC in its monthly report predicted a contraction of one million bpd to an average of 84.61 million bpd.
OPEC and the IEA have both warned under-investment could eventually drive prices higher, but the two groups interpret the near-term risks differently.
"With continued economic deterioration and demand erosion as well as the impending low demand season, there is a likelihood of renewed pressure on prices," OPEC's report said.
Oil prices, it said, needed to stay at levels that "support energy investment across the supply chain to help sustain longer-term economic growth".
"These are the main issues under consideration when the OPEC conference meets on March 15," it said.
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 Alex Lawler, Simon Webb and Sarah Marsh in Vienna and Jane Grieve and Christopher Johnson in London, Writing by Barbara Lewis; editing by William Hardy)