Updated from 11:54 a.m. ESTOil prices fell Monday after Nigerian militants released their last remaining hostages, relieving market concerns over tightening crude supplies. Light, sweet crude slipped 10 cents to close at $64.16 a barrel on the Nymex. Oil rose 6% last week to settle at $64.26 on Friday thanks to supply disruptions in Nigeria and ongoing refinery maintenance. "It takes a lull of three to four days when Iran doesn't say anything and nothing happens in Nigeria for a bull to sit down and take a break," says Guy Gleichmann, president of United Strategic Investors Group, a commodity brokerage firm in Hollywood, Fla. "That hasn't happened." On Monday, a Nigerian rebel group released one British and two American oil workers they had taken hostage five weeks ago and vowed to end kidnapping. Still, the group, which calls itself the Movement for the Emancipation of the Niger Delta, vowed to continue its campaign of violence against the country's oil industry. Thus far, Nigeria daily crude output is down 26% to 1.6 million barrels due to attacks on oil installations. Italian oil firm Eni ( E) said on Thursday it could not guarantee crude exports from its Brass River terminal, which processes 200,000 barrels per day, because of a pipeline attack last week. The conduit carries 75,000 barrels per day, but is expected to be operating by the end of this week. Despite the hostage release, Shell ( RDS.A) said it would not resume operations in the West African country until the safety of its workers could be guaranteed. The oil giant has lost 455,000 barrels of crude per day in Nigeria because of the attacks. The Chinese government increased diesel and gasoline prices 3% to 5% this weekend in an attempt to cool the country's soaring demand for oil. The increase was the first in eight months. GDP growth was also forecast to come in at 8.7% to 8.9%, short of the expected 15% to 20% growth the oil markets had expected.