(Adds positioning in natural gas, gasoline) By Stephanie Kelly NEW YORK, Sept 28 (Reuters) - Hedge funds slightly increased their bullish wagers on U.S. crude in the week to Sept. 25, data showed on Friday, as Saudi Arabia and Russia ruled out any immediate increase in production, raising concerns about supply shortages. The speculator group raised its combined futures and options position in New York and London by 3,728 contracts to 346,566 during the period, the U.S. Commodity Futures Trading Commission (CFTC) said on Friday. Brent crude speculators raised their net long positions by 28,465 contracts to 496,343 in the week to Sept. 25, the highest in four months, data from Intercontinental Exchange showed. Oil prices posted fresh four-year highs in the period after top global producers ended a meeting with no formal recommendation for any additional supply boost to counter falling supply from Iran. U.S. sanctions affecting Iran's petroleum sector, due to come into force in November, are expected to tighten global crude supply. JPMorgan said U.S. sanctions on Iran could lead to a loss of 1.5 million barrels per day (bpd), while commodities trader Mercuria warned that as much as 2 million bpd could be knocked out of the market. Speculators have been more bullish on global benchmark Brent futures compared with U.S. crude because surging domestic output is expected to keep a lid on prices. U.S. crude production rose 269,000 bpd to a record 10.964 million bpd in July, the U.S. Energy Information Administration said in a monthly report on Friday. However, there are signs of a slowdown as production has outpaced infrastructure in key basins in the country. New drilling has stalled in the third quarter, data showed, with the fewest additions in a quarter since 2017 due to pipeline constraints in the nation's largest oil field. Meanwhile, natural gas speculators in four major NYMEX, ICE markets raised their net long positions by 45,590 contracts to 160,597 in week to Sept. 25. In refined products, money managers raised bullish wagers by 12,025 contracts to 91,131 during the period, the highest level in about a month. (Reporting by Stephanie Kelly Editing by Tom Brown and Sandra Maler)