(Reuters) - Oil and gold edged lower as a firm dollar took the shine off most commodities on Monday, while uncertainty about the global economy limited any buying interest despite two of the sharpest sell-offs since the 2008 crisis in the past two weeks. Comments by U.S. President Barack Obama that a failure to raise the nation's debt limit could lead to a worse financial crisis and economic recession than 2008-09 and a potential euro zone debt restructuring also weighed on sentiment of investors still divided on where commodity prices were headed. Tight global supplies had boosted commodity prices before May but market players have been increasingly shifting focus to the outlook for weaker demand. "It does look that the prospect of weaker demand in advanced economies has now taken over as the core focus of the markets in terms of triggering price changes," said Ben Westmore, commodity economist at National Australia Bank. U.S. crude oil futures dropped more than a dollar to below $99 a barrel and Brent eased 58 cents to $113.25 by 12:45 a.m. EDT on a stronger greenback and as Obama's comments stoked concerns about demand in the world's No. 1 oil consumer. In remarks recorded last week and broadcast by CBS News on Sunday, Obama repeated his stance that Republicans should not link the debt ceiling decision to spending cuts as part of deficit-reducing measures. "If investors around the world thought that the full faith and credit of the United States was not being backed up, if they thought that we might renege on our IOUs, it could unravel the entire financial system," Obama said. The euro slid to a six-week low against the dollar and a two-month trough against the Japanese yen and is seen testing pivotal support areas after IMF chief Dominique Strauss-Kahn was charged with sexual assault on the weekend, increasing uncertainty on aid for Greece and other indebted euro zone countries. Euro zone finance ministers are likely to back a bailout package for Portugal on Monday, with new conditions set by Finland. The meeting was also expected to pressure Greece to announce more austerity steps to secure further emergency funding. The firmer dollar also weighed on gold and silver. Spot gold dipped 0.2 percent to $1,490.11 an ounce and spot silver shed 1.9 percent to $34.61. London copper dropped 0.4 percent to $8,758 a tonne. "We believe volatility will persist in near term, although we are still constructive toward the long-term outlook for most metals as supply will remain constrained and demand steady," Morgan Stanley said in a research note. Outperforming the market were corn and wheat which jumped more than 2 percent amid supply concerns. Chicago corn rose 13- cents to $6.95- a bushel on talk of renewed Chinese buying of U.S. corn and worries that delays to plantings of the U.S. corn crop this year may cut yields, analysts said. Corn's gain spilled over to wheat which climbed 15-3/4 cents to $7.43- a bushel ahead of a U.S. weekly planting progress report due later on Monday expected to show crops in the southern plains continued to be hit by drought.

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