CARACAS (Reuters) - German airline Lufthansa said on Monday it will reduce service to Venezuela by 40 percent starting next month amid an ongoing dispute about repatriation of revenue from ticket sales. Carriers, including American Airlines, Delta and United, have in recent weeks reduced flights to Venezuela over complaints they have not been able to obtain hard currency through the OPEC nation's currency control system. Lufthansa, Europe's largest airline by revenue, will cut flights to 3 per week from 5 per week as of Aug. 3, it confirmed to Reuters by email, citing "operational reasons." Aerolineas Argentinas as of Aug. 8 will reduce available seats by 17 percent and Air Europa of Spain will cut flights to Caracas by 57 percent, the association said. Reuters was unable to obtain comment from either airline. Venezuela requires airlines to sell tickets using the local bolivar currency. But carriers say they are not receiving approval from the exchange control board to convert those earnings into dollars. Airlines have an estimated $4 billion in revenue stuck in the country because of approval delays, according to the International Airline Transport Association. (Reporting by Diego Ore and Brian Ellsworth; Editing by Dan Grebler)