MOSCOW, Jan 18 (Reuters) - Gas supplies via the Yamal-Europe pipeline, which usually sends Russian gas west into Europe, increased on Tuesday morning eastward from Germany to Poland, running in a reverse mode for a 29th successive day, data from German network operator Gascade showed.

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 Tuesday morning were above 9.4 million kWh/h, up from around 7 million kWh/h on Monday and the weekend, data from the Mallnow metering point on the German-Polish border showed.

It is not clear when the pipeline will revert to westbound flows into Germany. A source close to Russian gas exporting monopoly Gazprom 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.

Russia has denied accusations from a number of European policymakers that it was withholding gas supplies to pressure German and European authorities to give a green light to the newly-built Nord Stream 2 pipeline.

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 287,423 megawatt hours (MWh) on Tuesday.

That level was steady with nominations seen so far in January but down sharply from December levels and 60% below levels seen a year ago.

(Reporting by Vladimir Soldatkin, additional reporting by Jason Hovet in Prague; Editing by Kim Coghill)