Outright bunker fuel prices are likely to rise in August-September on the back of tight availabilities from maintenance at Exxonmobil's Antwerp refinery, sources said.

"The refinery has already stopped loading for spot deliveries," one bunker market source said Wednesday.

"It will be supplying on contract basis only," he added.

ExxonMobil's 307,500 b/d Antwerp refinery will be fully shut for maintenance between mid-September and early November, traders said late last week.

Operator ExxonMobil wasn't available for immediate comment on the matter. The plant already underwent extensive works in March and April this year.

Traders said there has so far been no impact on high sulfur delivered bunker premiums, which were pegged Tuesday at $2/mt over FOB 3.5% ex-wharf barges.

According to Platts data, 380 CST HSFO was assessed at $600.50/mt Tuesday.

However, Antwerp traders said delivered bunker premiums would likely rise towards the start of the refinery maintenance period.

"Bunker fuel will be expensive and difficult to find towards when the Exxonmobil refinery goes into maintenance in six to eight weeks in Antwerp," one source said.