VIENNA--Some Central European countries continued to report declines in Russian gas deliveries on Monday as tensions between Ukraine and Russia persist in the run-up to winter.

Austria's energy regulator E-Control said gas supplies were down by 25% on Friday--the largest decrease for the Alpine country so far. Deliveries did increase the next day and "on Saturday it was only [down] 20%," E-Control's Executive Director Martin Graf said.

For this time of year, due to weather and the filling of storage capacity, declines of between 10% and 15% are within the range of normal, he added.

Slovak natural gas distributor SPP AS said that flows of Russian gas to the country were down by 20% from contracted levels on Monday. Last week flows of Russian gas into Slovakia were down by as much as 25%.

Polish natural gas operator Gaz System said Russia's Gazprom continues to deliver less gas than what is ordered.

SPP, like its Austrian peer, said that there were no supply disruptions to domestic gas customers despite the declines in gas flows from Russia.

Austria's gas storage capacity is full and demand for gas is low due to the relatively warm weather, Mr. Graf said.

"It isn't a worrying situation, but of course we need to see if the decreases continue in the near future," he added.

Mr. Graf declined to comment on how long Austria could meet demand should Russian gas deliveries cease completely, referring instead to a coming stress test by the European Commission on such a scenario.

Earlier this month, a spokesman for Austria's largest oil and gas company OMV AG (OMV.VI) said Austria's storage capacity was roughly equivalent to a year's demand of gas.

The Austrian regulator has contacted Russia's Gazprom about the decreases, but "there is so far no answer," Mr. Graf said.

The fall in gas supplies comes as tensions between Russia and Ukraine continue and the two remain at odds regarding payments for gas. Moscow is demanding Kiev pay $385 per thousand cubic meters for its gas, a price the economically stressed Ukraine is unwilling to pay. Russia shut off its gas deliveries to Ukraine in June.

Both Slovakia and Poland, and to a lesser degree Hungary, are delivering gas to neighboring Ukraine after Gazprom stopped shipping gas there in June, citing several billion dollars" worth of unpaid bills.

Despite the reduced flows to Poland, the country continues to supply Ukraine with gas via Germany and the Czech Republic.

Slovakia's pipeline operator Eustream also said Monday that its flows to Ukraine are in a "normal mode" and aren't affected by reduced flows into Slovakia for domestic use.

Russia, Ukraine and the European Union are due to restart talks on gas supplies to Ukraine on Friday, Sept. 26, in Berlin.

Patryk Wasilewski in Warsaw contributed to this article.

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