LONDON, Aug 27 (Reuters) - Gasoil and diesel differentials were steady on Tuesday, while gasoil and crude oil futures soared as Western powers appeared poised to strike the Syrian president's forces in response to a chemical weapons attack against civilians last week. Western officials told the Syrian opposition to expect a strike against President Bashar al-Assad's forces within days, according to sources who attended a meeting between envoys and the Syrian National Coalition in Istanbul. Diesel has been underpinned by outages at refineries in the United States and Canada, Motiva and Irving, respectively. Fuel oil cracks have been under pressure due to lower Asian demand. "Demand in Japan has been weak due to an increase in the burning of coal at the expense of fuel oil, while increased shipments from Europe to Asia have seen inventories in Singapore move above average," Deutsche Bank said in an oil note on Tuesday. Rosneft has launched a new crude distillation unit (CDU) at its Tuapse refinery that will almost triple capacity at the plant, which exports the bulk of its oil products to Mediterranean markets, company sources told Reuters on Tuesday. GASOIL * One barge of 0.1 percent gasoil barge traded at $3 a tonne fob ARA below September ICE gasoil futures, in line with Friday's offers and bid. * No 50 ppm gasoil barge traded but a bid surfaced at a $17 a tonne fob ARA premium to September ICE gasoil futures. * The ICE September gasoil futures was up $21.50 at $961.75 a tonne at 1623 GMT. * The ICE gasoil crack was down slightly at $15.15 a barrel. * The contango for September/October widened slightly to $1.25 from $1 on Thursday. DIESEL * Eleven diesel barges traded at premiums to September ICE gasoil futures of $21.50-$23 a tonne fob ARA, in the same range as Friday at $22-$23 a tonne. * BP, Vitol, Noble, Litasco and Argos sold to Shell, Mocoh and Totsa. JET FUEL * No jet fuel barges traded. Totsa bid for a barge at $70 a tonne fob ARA above September ICE gasoil futures while KLM and BP offered at $69 a tonne above September futures and CCM plus $2.50 a tonne, respectively. * No cargoes traded. FUEL OIL * Barges of low sulphur fuel oil with 1 percent sulphur content traded at $600-$601 a tonne fob ARA, down from $612 a tonne fob ARA, down from $615 a tonne.