SINGAPORE, Jan 14 (Reuters) - Medium sour crude exports from the Middle East loading in March have sold at premiums after strong demand from Chinaoil tightened supplies, trade sources said on Thursday.

The trading arm of PetroChina has bought nine Upper Zakum cargoes during the Platts Market on Close (MoC) process so far this month, they said.

The purchases tightened supplies and pushed up spot values of similar grades in the market, traders said.

On Wednesday, Japanese refiner Fuji Oil bought two cargoes of Banoco Arab Medium and one Qatar Marine cargo for March loading via a tender at premiums of 30-35 cents a barrel above the crudes' respective official selling prices (OSPs), they said.

Oman crude futures on the Dubai Mercantile Exchange were also at their strongest in three weeks versus Dubai swaps on Wednesday.

Banoco Arab Medium last traded at discounts of more than 20 cents for cargoes loading February, while spot differentials for Qatar Marine were in a wide range.

More Banoco Arab Medium will be sold through a tender issued by Bahrain Petroleum Company. The tender will close on Jan. 15 with bids valid until Jan. 18.

(Reporting by Florence Tan; Editing by Joseph Radford) ((; +65 6870 3497; Reuters Messaging: