The European Union said it remained concerned over a gas deal between Russia and Poland.
The EU said terms for access to a gas pipeline under a deal between Russia's Gazprom and Poland's PGNiG may breach European competition law.
The two companies struck an agreement in January on transit volumes and rates for gas crossing Poland en route to Europe through the Yamal-Europe pipeline.
The deal will run through until 2045, but Poland has yet to sign the contract awaiting advice from Brussels.
The EU has relayed to Moscow and Warsaw concerns over the management of the pipeline by joint venture EuRoPol Gaz, which is 96 per cent owned by Gazprom and PGNiG, with talks scheduled for next week.
Relations were strained for years prior to the agreement being announced, with negotiations soiled by PGNiG accusations that Gazprom was seeking steep tariff hikes and undue influence over the running of the pipeline.