By Wiktor Szary and Jakub Iglewski

WARSAW (Reuters) - Poland believes Russia may be seeking to extend its ban on imports of food products from the European Union to goods in transit through Russia, Deputy Prime Minister Janusz Piechocinski said on Wednesday.

"We are receiving the first indications that Russia's customs and other authorities are trying to widen the embargo also to the transit of goods," Piechocinski told reporters, but did not clarify where those indications were coming from.

"I will personally intervene on various diplomatic levels in this case," said Piechocinski, adding that any "transport war" would harm relations between the EU and a Customs Union of former Soviet states that was set up by Russia, Kazakhstan and Belarus.

Russia banned most fruit and vegetable imports from Poland last month and extended the embargo to the rest of the European Union this month in retaliation for Western sanctions against Moscow over Russia's role in the Ukraine crisis.

Piechocinski has said previously that the ban could have 0.6 percentage points off Poland's gross domestic product this year.

Jan Buczek, head of the Polish Association of International Road Transport Carriers said that Russia was now blocking the transit of some embargoed goods to other countries, such as Kazakhstan, Mongolia and the Caucasian countries.

"They are even stopping Polish carriers with cargos which are not embargoed," he told Reuters.

Customs Union members Kazakhstan and Belarus did not join in Russia's ban on foodstuffs from the EU.

In theory, Polish exports could be shipped to Belarus or Kazakhstan and from there re-exported to Russia, side-stepping Moscow's embargo.

Russia's Customs Service denied any wrongdoing. "We allow transits (of banned food products)," Vladimir Zubkov, a spokesman for the Customs Service, told Reuters when asked to comment on the statement by the Polish Association.

(Reporting by Wiktor Szary and Jakub Iglewski; Additional reporting by Polina Devitt; Writing by Marcin Goettig and Wiktor Szary; Editing by Susan Fenton)