Oil firmed towards $48 a barrel on Tuesday, although staying within the trading range it has held so far this month, ahead of United States inventory data expected to show a build in crude oil stocks. Data from industry group the American Petroleum Institute (API) will be closely watched later on Tuesday as a preliminary Reuters poll showed analysts forecast a 500,000 barrel increase in domestic crude stocks last week. U.S. light crude for April delivery rose 63 cents to $47.98 a barrel by 1039 GMT. The contract had settled $1.10 higher at $47.35 on Monday. London Brent crude gained 66 cents to $47.12.