The European Commission is proposing to extend EU internal energy market rules to cover offshore gas pipelines, which it sees as undercutting EU efforts to reduce dependence on Moscow and its support for Kyiv, Reuters said.

"I would like to encourage you to proceed swiftly with this proposal," Tusk said in the letter, seen by Reuters and dated November 10, to the heads of the bloc's 28 nations. "Time is of the essence, if we are to achieve the objectives of our Energy Union."

Read alsoGazprom says new U.S. sanctions pose risks for gas pipelines bypassing UkraineTusk has sided with member nations such as Poland and the Baltic states that argue Nord Stream 2 would increase the EU's dependence on Russia's state gas export monopoly Gazprom, which already supplies about a third of the bloc's gas.

But Germany, the end-point for Russia's plans to double the gas it could pump under the Baltic Sea to Germany, bypassing traditional routes via Ukraine, believes Brussels should not get involved.

EU regulators say they want the rules in place by the end of 2018 – early enough to apply to the Nord Stream 2 pipeline due to be completed by the end of the following year.

Even with the changes, EU regulators say they may need to seek talks with Russia as it cannot impose its law on the stretch of the pipeline that is outside its territory.

The Commission has also appealed to EU nations for a mandate to negotiate with Russia over objections to the pipeline.

Five European energy firms are financing the 1,225 km (760 mile) pipeline to carry 55 billion cubic meters of gas per year: German energy groups Uniper and Wintershall, Anglo-Dutch group Shell, Austria's OMV and France's Engie.