* Euro gains, ECB loosens lending rules

* Europe stays in focus ahead of EU summit

* Oil supply ample, OPEC pumps more than target (Recasts, updates prices, market activity and adds details)

By Gene Ramos

NEW YORK, June 22 (Reuters) - Brent oil futures rose above $91 a barrel on Friday, rebounding after hitting an 18-month low, and U.S. crude futures surged nearly 2 percent as a potential storm threatened to disrupt production in the Gulf of Mexico.

An action by the European Central Bank to ease collateral requirements -- a move designed with Spain's woes in mind, caused the euro to rally early and also supported oil futures.

After falling about 4 percent on Thursday oil futures regained some composure as some buyers returned after the market had come under oversold conditions, analysts said.

In London, August Brent crude was up $1.92 at $91.15 by 2:50 p.m. EDT (1850 GMT), having hit a session high of $91.70. It rebounded from a session low of $88.49, the lowest since December 2010. It is on course to end the week down more than 6 percent.

U.S. August crude settled at $79.76, gaining $1.56, after rising to a session high of $80.37. It slid as low as $77.56, the lowest since Oct. 5, 2011, after dropping 4 percent on Thursday. For the week, front-month U.S. crude fell $4.27, or 5.08 percent, the biggest weekly loss since the week to June 1, when prices fell 8.4 percent. U.S. crude has posted two straight weekly losses.

"The ECB's action will add liquidity to the system, and that is helping push up Brent futures. The oil markets are rebounding from oversold conditions, though investors are cautions because the market is well supplied," said Phil Flynn, an analyst at Price Futures Group in Chicago.

At the day's lows, Brent has fallen nearly $40 from the year's high of $128.40 hit in March. U.S crude has dropped $33 from its 2012 of $110.55 also struck in March.

In the Gulf of Mexico, the largest U.S. offshore oil port and Murphy Oil Corp. began evacuating non-essential personnel from their operations.

The U.S. National Hurricane Center said a low pressure system in the Gulf, home to 20 percent of U.S. oil production and 6u percent of natural gas output, had a 70 percent chance of developing into a tropical cyclone over the next two days.

The Relative Strength Index, a closely watched technical signal, showed crude futures in oversold condition -- usually a sign that a rebound may be coming. For Brent, the index was at 23, after hitting 14 on Thursday. U.S. crude's RSI was at 30, after posting at 22 on Thursday. The 30 level is the threshold that indicates the onset of oversold conditions, which both contracts last began hitting in late May.

"Technical indicators show the market is a little bit oversold, so there could be some short-covering around," said Tony Machacek, oil futures broker at Jefferies Bache.

"It has been a long fall, driven by global economic slowdown and oil fundamentals, such as weaker demand from China," he added.

Early on Friday, oil and other commodities and global equities came under pressure after the ratings agency Moody's downgraded the credit ratings of 15 of the world's biggest banks to reflect potential losses from volatile capital markets.

On Thursday, oil futures tumbled as data showed U.S. factory output grew at its slowest pace in 11 months in June, business activity across the euro zone shrank for a fifth straight month and Chinese manufacturing contracted for an eighth month.

STRONG SUPPLY

While oil demand prospects are dimming, supply of oil remains ample. The Organization of the Petroleum Exporting Countries is pumping about 1.6 million barrels per day (bpd) more than the demand for its oil and its own supply target, OPEC figures show.

Much of the extra oil has come from top exporter Saudi Arabia, as well as from an export capacity expansion in Iraq and a recovery in Libyan output.

At its meeting last week, OPEC agreed to keep its oil output limit at 30 million bpd, with several members urging the Saudis to cut back supplies to reach the target.

"We are heading for a weak third and fourth quarter, so prices could go a lot weaker," said Leo Drollas, chief economist at the Centre for Global Energy Studies. "The Saudis at the end of the day will have to cut back themselves."

Amid the festering euro zone debt crisis, the leaders of Germany, France, Italy and Spain agreed on Friday on a 130 billion euros ($156 billion) package in a bid to revive economic growth in Europe. However, they differed on whether to use euro bonds to ease the region's debt troubles. (Additional reporting by Alex Lawler in London and Luke Pachymuthu in Singapore; Editing by David Gregorio and Marguerita Choy)