* Brent crude around highest in three months * North Sea maintenance, Gulf of Mexico storm support * U.S. jobless claims better than forecast * OPEC downbeat on demand for 2013 (Updates prices) By Claire Milhench and Simon Falush LONDON, Aug 9 (Reuters) - Oil gained on Thursday, supported by better than forecast U.S. employment and trade data, and on worries about supply disruptions in the Gulf of Mexico and lower Brent output. However gains were kept in check by a downbeat assessment of demand outlook for 2013 by the Organization for Petroleum Exporting Countries. Brent crude for September delivery was up 41 cents at $112.55 a barrel by 1254 GMT, and was heading for its highest since early May. U.S. crude was up 51 cents at $93.86 a barrel. The oil price remains underpinned by supply restrictions in the North Sea due to maintenance and port closures in the Gulf of Mexico as the hurricane season gets underway. "The general mood is bullish - any dip is still being used as a buying opportunity," said Carsten Fritsch, an energy analyst at Commerzbank in Frankfurt. "Given the supply risk, with falling North Sea output and the closure of three oil ports in Mexico, all this should lend support to prices." North Sea oil output will plunge 17 percent in September due to maintenance at the Buzzard oilfield and natural decline, adding to signs of a shortage that may artificially lift prices of Brent. As a result, Harry Tchilinguirian and Gareth Lewis-Davies, commodities strategists at BNP Paribas, thought Brent's premium to U.S. crude could widen beyond the $18-$19 seen on Thursday. "At this juncture, expectations surrounding the impact of lower North Sea production in September are likely mostly embedded in the front month Brent price. Yet, we cannot exclude the possibility that Brent's premium may rise further," they said in a note. Brightening the demand outlook somewhat, the number of Americans filing new claims for jobless benefits unexpectedly fell last week, government data showed on Thursday, suggesting a modest improvement in the labour market. Also feeding into a slightly more optimistic assessment for the demand outlook, the U.S. trade deficit in June was the smallest in 1-1/2 years as lower oil prices curbed imports, according to government data. Tropical Storm Ernesto has moved into the southern Bay of Campeche in the Gulf of Mexico, where the country's main oil operations are located. Mexico closed its three major oil export ports on the Gulf. Adding to general feeling of tighter supply, an industry source said Saudi Arabia had cut its crude output in July to 9.8 million barrels per day (bpd), down from 10.1 million in June. Saudi has been pumping at a higher rate since spring to compensate for the loss of Iranian crude due to a Western embargo. OPEC DOWNBEAT However implying significant downside pressure once supply constraints ease, the Organization of the Petroleum Exporting Countries (OPEC) pointed to risks for oil demand growth. OPEC said it expected world oil demand to expand by 810,000 barrels per day (bpd) next year, unchanged from its previous forecast, although the odds suggested demand could undershoot. But there were some hopes that further monetary easing following disappointing Chinese data could help support demand. This showed annual growth in Chinese factory output slowing to its weakest in more than three years in July, and retail sales missing market forecasts. In addition, annual consumer inflation has fallen to a 30-month low in July, suggesting that the central bank has ample scope to ease policy. China's refinery throughput inched up 1.1 percent in July, reversing a run of declines for three straight months, but this was the second-lowest level this year as demand stayed tepid in the world's second-largest oil consumer. (Additional reporting by Florence Tan in Singapore; Editing by William Hardy)

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