LONDON, Aug 23 (Reuters) - Diesel barge outright prices in northwest Europe edged to fresh six-month highs on Friday as concerns of tighter regional supplies due to U.S. refinery outages rose, traders said. Diesel imports from the United States were expected to decline by around 15 percent, or 130,000 tonnes, over the next month due to extensive shutdowns at Motiva Enterprises' two largest refineries. "We're seeing the last bits of summer demand before the intermediate grades kick in. The Motiva outages will be reducing imports, too," a trader said. Demand in the Med was set to increase in September after Kuwait Petroleum International (KPI) issued a tender to buy two 30,000 tonne cargoes of 10 ppm ultra-low sulphur diesel for delivery into the port of Naples, Italy in the first half of September. Tupras, Turkey's sole oil refiner, also issued a tender to buy 30,000 tonnes of straight-run high sulphur gasoil for delivery in mid-September, traders said. The cargo of 0.1 percent and 0.2 percent gasoil is to be delivered to the ports of Izmir or Izmit on a cfr basis. Russia's energy ministry told oil companies to boost gasoline and diesel stocks following a spike in domestic prices but refrained from ordering an outright export ban, according to a document and an official. It was unclear if the move would impact Russian diesel exports to Europe and the Mediterranean. GASOIL * No barges of 0.1 percent gasoil barge traded. Offers and bids were seen at discounts of $3 a tonne fob ARA to the September ICE gasoil futures, but the volumes did not match. * One 50 ppm gasoil barge traded at premiums to September ICE gasoil futures of $17 a tonne fob ARA, down $2 a tonne from Thursday. * The ICE September gasoil futures was up $7.25 at $943.50 a tonne at 1553 GMT. * The ICE gasoil crack was down slightly at $15.76 a barrel from $16.16 a barrel. * The contango for September/October widened out to $1 from 75 cents on Thursday. DIESEL * Six diesel barges traded at premiums to September ICE gasoil futures of $22-$23 a tonne fob ARA, unchanged from Thursday. * BP and Vitol sold to Shell. JET FUEL * No jet fuel barges traded with no offers seen. Morgan Stanley made one bid and Shell bid for four barges at $67 a tonne fob ARA above the gasoil futures. * No cargoes traded. Bids came in from BP, Morgan Stanley and Shell at $69-$70 a tonne cif NWE above the September gasoil futures. Vitol offered three cargoes. FUEL OIL * Barges of low sulphur fuel oil with 1 percent sulphur content traded at $612 a tonne fob ARA, down from $615 a tonne. * High sulphur fuel oil barges with 3.5 percent sulphur content traded at $593-$594 a tonne fob ARA, compared with $593.25-$596 a tonne on Thursday.