Oil prices slipped, adding to Friday's falls, as high crude stockpiles in the US continued to weigh on the market, but falls were tempered by news of another oil pipeline attack in key producer Nigeria. At 12.00 pm, May-dated Brent contracts were down 29 cents at 62.97 usd, after sliding 95 cents to end trade at 63.26 usd Friday. Meanwhile, April-dated US light crude futures were down 35 cents at 62.42 usd. Prices were up earlier in the session on weekend news Nigerian militants blew up an ENI-operated oil pipeline, cutting off some 67,000 bpd of output and bringing Nigeria's total outage to 622,000 bpd or 25 pct of national output. Offsetting the Nigerian output, however, is the continued build in US crude supplies, which are currently at 7 year highs and are probably set to grow even further near term. Calyon analyst Mike Wittner said the spring refinery maintenance season is the key dynamic influencing the market at present as low refinery runs are leading to lower demand for crude. "With maintenance causing low refinery runs, the bottom line is that crude stocks are building and product stocks are drawing, and this general state of affairs will continue for the next 30 days," he said. The build in crude stocks will therefore weigh on prices near term, and as it is accompanied by low demand, it is possible prices will correct down to 55-58 usd, according to Wittner. "However, if (the correction) doesn't happen in the next 30 days, it won't happen this spring, because both crude and product demand will be on the upswing after that," he said. Product demand is seasonally weak at present because the winter heating season is over while the peak demand summer driving season -- when American drivers take to the roads on annual vacations -- has not yet begun. However, gasoline demand is holding up well for the time of year and traders are concerned that with stockpile levels already falling, supplies will be stretched tight in summer. The concerns are exacerbated by the US Energy Department's new greener fuels specifications requiring refiners to switch to producing a new type of gasoline blended with ethanol. Investec analyst Bruce Evers said overall, the risk for prices is still "very much on the upside", with questions hanging over the gasoline market and geo-political tensions raging in key producers Iran and Nigeria. Officials from the five veto-wielding members of the UN Security Council and Germany are meeting in New York today to discuss ways of curbing Iran's nuclear programme -- seen by Western nations as a front for a weapons drive. Traders fear Iran, the world's fourth largest crude producer, could use oil as a political weapon to counter-act the threat of security council sanctions.