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By Katya Golubkova and Denis Pinchuk

MOSCOW, Oct 13 (Reuters) - Saudi Arabia has started supplying crude oil to Poland, the head of Russia's biggest oil company Rosneft said on Tuesday, becoming another Middle Eastern producer to enter a market traditionally dominated by Russia.

A global battle is underway among oil exporters for market share, with producers with the deepest pockets, such as Saudi Arabia, using low prices to enter new markets, often at the expense of Russia.

If Russia appears to be losing that battle for market share, it could have an impact on decisions made about crude production at a meeting of oil exporters club OPEC in December.

"We are working under conditions of tough competition," Rosneft Chief Executive Igor Sechin told an investor conference in Moscow.

"Saudi Arabia has entered the Polish market for the first time, with deliveries via Gdansk," he said, referring to the port on the Baltic Sea.

"They are actively dumping, which is an element of the changes in world prices. Without doubt, the battle for markets is at this stage one of the key factors."

"We have to make every effort to prevent a decrease in our share of supplies," Sechin said.

No details on the volumes of Saudi crude sent to Poland were given.

In another indication of the shifting balance in the global market, Hungary has increased oil imports from Iraq's Kurdistan region, displacing Russian crude, according to trading sources.

Global oil prices have more than halved since June last year. Saudi Arabia, the top exporter, has refused to cut production.

It says higher-cost oil producers would ultimately reduce output and help the market to rebalance.

But U.S. oil output has proved more resilient than expected and Russia has continued to increase production in the past few months, further worsening the global glut.

(Writing by Christian Lowe, editing by Louise Heavens) ((Polina.Devitt@thomsonreuters.com; +7 495 775 12 42; Reuters Messaging: polina.devitt.reuters.com@reuters.net))