Poland, Germany, Ukraine, Slovakia and other countries halted oil imports via the Druzhba pipeline last week after finding contaminants that can damage refinery equipment.

The released reserves are to ensure continuity of refinery processing at normal levela, a ministry spokeswoman said in an email to Reuters on Thursday.

The ministry decided on April 26 to release 500,000 tonnes of stocks and on April 30 to free 300,000 tonnes.

"Both decisions were notified to the European Commission and the International Energy Agency," the spokeswoman said, adding that the ministry continues to monitor the situation.

The spokeswoman also said that it was up to Polish refiners PKN Orlen and Lotos to decide when and how much of the reserves they take.

Poland's two refineries - PKN's Plock and Lotos's Gdansk plant - refine an annual 33.4 million and 10.8 million tonnes of crude respectively.

On Tuesday Lotos said it was using the strategic stock, though it was unclear whether it tapped the reserves on Wednesday and Thursday.

An industry source said the Gdansk refinery received an oil shipment on Wednesday and that it does not have to use the reserve stock every day.

PKN Orlen did not respond to Reuters' question on whether it is refining oil from the strategic reserves. Last week it said it had no plans to do so.

"We discuss with our clients how to return to normal in the fastest way possible and how to clean their commodity," said a spokeswoman at state-owned pipeline operator PERN.