MOSCOW, Jan 21 (Reuters) - Gas supplies via the Yamal-Europe pipeline, which sends Russian gas west into Europe, have declined from Germany to Poland, running in a reverse mode eastward for a month, data from German network operator Gascade showed on Friday.

Since Dec. 21, the link between Poland and Germany had been operating in reverse mode, putting upward pressure on European gas prices.

The pipeline usually accounts for about one-sixth of Russia's annual gas exports to Europe and Turkey.

The reverse flows seen on Friday morning were above 11.7 million kWh/h, down from 13.1 million kWh/h for the past 24 hours, data from the Mallnow metering point on the German-Polish border showed.

According to preliminary bids, the flows are due to stay at volumes of over 11.7 million kWh/h until the early hours of Saturday.

It is not clear when the pipeline will revert to westbound flows into Germany. A source close to Russian gas exporting monopoly Gazprom has said the company is expected to switch flows at some point this month, as Gazprom has paid for westbound volumes for January.

Gazprom has not booked any capacity to pump gas to Europe through the Yamal pipeline next month, underscoring a sharp drop-off in Russian exports to the region so far this year.

Capacity nominations for Russian gas flows from Ukraine to Slovakia via the Velke Kapusany border point, another major route for Russian gas to Europe, stood at 288,086 megawatt hours (MWh) on Friday.

That level was in line with nominations so far in January but down sharply from December levels and 60% below a year ago. (Reporting by Vladimir Soldatkin; Additional reporting by Jason Hovet in Prague; Editing by Christopher Cushing)