The planning companies behind an expansion to a natural gas pipeline in the Baltic Sea said they're exploring their options amid Polish anti-trust concerns.

Russian natural gas company Gazprom and a consortium of European energy companies are planning a second string for the twin Nord Stream natural gas pipeline running through the Baltic Sea to Germany.

The Polish antitrust authority, known by its acronym UOKiK, said the enhanced role for the Russian energy company through Nord Stream II "might lead to restriction of competition."

The project consortium said it was notified of the concerns Wednesday and studying the ruling.

"The applicants are currently preparing their joint response to the statement of objections," the project consortium said in an emailed statement.

Gazprom signed shareholder agreements on the development of the second phase of the twin Nord Stream pipeline system last year with counterparts that include Austrian energy company OMV, French company ENGIE and Royal Dutch Shell. Under the proposed expansion, two more lines would be added to the existing network, roughly doubling the pipeline's net capacity.

Gazprom has said the project would be commissioned before late 2019. Polish officials, concerned that Russia already controls most of the country's gas supplies, said the pipeline would only strengthen its hand.

Poland imports most of its oil and natural gas from Russia.

The European Union has expressed concern about Russia's control over the regional market as the Russian gas company typically controls both the transit networks and the reserves they deliver.