Volumes were light, however, as UK and Japanese markets observed a public holiday. "Nigeria is the lingering hotspot the markets will be focussing on," said MF Global analyst Ed Meir. "Here, the news over the weekend has been mixed; ExxonMobil said it has restarted 300,000 bpd of Nigerian production out of total of 800,000 sidelined earlier, but there are reports of fresh violence, as another pipeline explosion has shut in more oil production," he added. Fresh militant attacks in Nigeria, Africa's biggest producer, have forced oil major Shell to shut down more of its oil production. Nigerian militants attacked an oil ship off the coast of the west African country and took two people hostage, a military spokesman said Sunday. Shell accounts for about one-half of Nigeria's 2.1 million barrels-per-day output. "A few oil delivery lines are affected and some oil has spilled into the environment," a Shell spokesman said. New York-traded West Texas Intermediate crude for June delivery was up 33 cents to $116.65 a barrel. In London, Brent crude for June delivery was up 19 cents at $114.75. Trading volume in London was light as the United Kingdom enjoys a bank holiday. Elsewhere, prices were supported by tensions between Iran and the West. Iran said Monday it will reject any offer that violates its right to the full nuclear fuel cycle after world powers said they had prepared a new package to end the atomic crisis. Oil players fear the ongoing tension could result in Iran using oil as a bargaining chip. Oil rose 3 percent Friday on better-than-expected employment figures in the United States which raised hope the world's biggest economy might avoid a recession. The bombing by Turkish warplanes of Kurdish rebel bases inside Iraq and the strengthening of the U.S. dollar against the euro also supported oil prices.