Oil fell nearly two dollars Monday, unwinding gains made after a foiled al Qaeda attack on the world's biggest oil processing plant in Saudi Arabia.

No supplies were disrupted from the leading oil exporter but tension remained high.

Al Qaeda has warned of further attacks and Saudi forces Monday killed five suspected militants believed to be linked to Friday's suicide bomb attempt.

U.S. light sweet crude for April delivery settled $1.91 lower at $61, off a session low of $60.85. Brent crude futures fell $1.61 to $60.99 a barrel.

Prices had jumped more than $2 a barrel Friday when news of the Saudi assault added to nervousness about supply disruption in Nigeria, as well as tension between the West and Iran over its nuclear ambitions.

Analysts said gains would have been bigger but for high fuel stockpiles in the United States, the world's biggest energy consumer.

"If it weren't for the market's focus on short-term inventory data, the market would be a lot higher," said Kevin Norrish of Barclays Capital. "We're retracing, even if the move upwards was not as big as it could have been."

Oil analysts surveyed by Reuters Monday said they expected oil supplies in the U.S. rose last week by 900,000 barrels, while gasoline inventories slipped slightly.

U.S. oil supplies are already running about 10 percent higher than last year, while gasoline inventories are at their highest since June 1999.

Disruption from Nigeria, the world's eighth largest exporter has underpinned oil prices following militant threats to continue attacks on installations that have already forced the shut in of 455,000 barrels per day.

The situation in Iran, the world's fourth biggest exporter, also unnerved traders.

Tehran said on Sunday it had reached a "basic agreement" with Russia on jointly enriching uranium, but there was no immediate sign it would suspend home-grown enrichment to allay fears it is developing nuclear weapons.

The International Atomic Energy Agency meets on March 6 in Vienna to discuss its latest report on Iran, two days before a meeting of the Organization of the Petroleum Exporting Countries, also in Vienna, to review its output policy.

Most observers predict the producers' group will leave output unchanged, though some OPEC members have expressed concern the market is becoming over-supplied following a year of production at close to capacity.

Algeria's Energy and Mines Minister Chakib Khelil said at the weekend OPEC should leave output unchanged.

"Our position is that we should not change production," Khelil told a forum organised by state television.