Oil tumbled 6 percent on Thursday to a four-month low after the world's top consumers released emergency oil reserves for the third time ever, a surprise intervention to aid the struggling global economy.

The International Energy Agency announced it would inject 60 million barrels of government-held stocks in the global market, immediately increasing world supply by some 2.5 percent for the next month and sending prices spiraling, with U.S. crude prices erasing all of the year's gains.

The move shocked traders who had been expecting the IEA to give top exporter Saudi Arabia more time to make up for the supply shortfall following OPEC's failed meeting on June 8, when other members blocked Gulf efforts to hike output.

It comes after the Saudis said they would increase output so it suggests they think this might not be enough, said Helen Henton, head of commodity research for Standard Chartered Bank. I think it will knock prices lower. I expect prices to be lower a month from now.

Goldman Sachs, whose oil price forecasts are closely watched by markets, said the release of the IEA oil could knock prices for Brent crude down by $10 to $12 a barrel.

Brent crude futures for August plunged by more than $8 after the news, before settling at a four-month low of $107.26 a barrel, down $6.95 for the day.

U.S. crude lagged the decline as traders bet the relaxation of European oil reserve requirements would have a more direct and immediate impact on London Brent.

August U.S. crude dropped $4.39 to settle at $91.02 a barrel, taking prices more than 20 percent below their post-2008 high above $114 in early May.

U.S. heating oil futures also slumped nearly 6 percent, down 17.32 cents to settle below the 150-day moving average.

BRENT HIT HARDEST

The deeper drop in Brent pushed the London grade's premium to U.S. oil to just over $16 a barrel, off $3 from Wednesday in late trade. As part of the action, the United States will release 30 million barrels of crude from its Strategic Petroleum Reserve, specifically light sweet oil.

Brent trade volumes surged to a record high, hitting 1.15 million lots trading in late activity, nearly 2-1/2 times the 30-day average. U.S. volume topped 920,000 lots traded, 30 percent over the 30-day average.

The Brent futures curve flattened on the news, with the premium of the August contract to the September contract to 9 cents, down from 45 cents on Wednesday and reflecting an increase in prompt supplies.

Oil markets were already down sharply ahead of news of the release, due to worries over global fuel demand following higher-than-expected U.S. jobless claims, forecasts of lower U.S. growth from the Federal Reserve and evidence of a slowdown in Chinese manufacturing. The economic concerns helped push investors out of gold, down 2 percent on the day, while other commodities showed smaller losses.

The sell-off also followed a move by the U.S. Federal Reserve on Wednesday to cut its growth forecasts for the world's biggest economy.

IEA EYES ECONOMIES

This supply disruption has been underway for some time and its effect has become more pronounced as it has continued, said the IEA. It said expectations were that Libyan production would remain off the market for the rest of 2011.

Greater tightness in the oil market threatens to undermine the fragile global economic recovery, it said.

The IEA release, at 2 million barrels per day (bpd) over the next 30 days, is more than the daily loss of Libya's 1.2 million bpd exports and comes despite a broad view that the world is not now short of crude -- although many analysts and agencies also agree that markets will tighten later this year.

The release includes 30 million barrels of light, sweet crude from the U.S. Strategic Petroleum Reserve -- the same quality that markets have lost due to the Libya disruption.

Against that backdrop, analysts said the use of the reserves now -- unlike the previous two releases, which immediately followed the first Gulf War and Hurricane Katrina -- signaled it may have been more concerned with tempering prices to aid a faltering economic recovery.

The move is significant, as it represents a reach by member countries for the remedy of last resort to high oil prices, said John Kilduff, a partner at Again Capital LLC. Clearly, the energy price spike is being cited as the reason for the economic slowdown and this is a reaction to that. The Libyan outage provides good cover.

(Additional reporting by Robert Gibbons, Gene Ramos, Antonita Devotta, Jonathan Leff in New York; Manash Goswami in Singapore and Christopher Johnson in London; editing by Lisa Shumaker)

Libya Energy