BRUSSELS, Dec 11 (Reuters) - European efforts to keep Russia shipping gas through Ukraine after the opening the Nord Stream 2 pipeline across the Baltic Sea are futile, the top U.S. energy diplomat said on Tuesday, saying Russia could not be trusted.

Germany, the main beneficiary and destination for the pipeline, says it has secured a promise from President Vladimir Putin that Russia will maintain gas shipments across Ukraine.

Washington and other opponents of the pipeline, which will double the capacity of the existing Nord Stream 1 route from next year, fear it will strip Ukraine of important transit revenues.

Ramping up U.S. pressure on Germany to withdraw support for the pipeline, U.S. energy envoy Francis Fannon said Russia’s actions against Ukraine in the Kerch Strait show it cannot be trusted.

Moscow is resisting international calls to release three Ukrainian ships seized last month in the Strait, which controls access to the Sea of Azov near the Crimea region that Moscow annexed from Ukraine in 2014.

“There is a problem because you are expecting to trust Russia to maintain and not cut off that gas,” Fannon, the U.S. assistant secretary for energy resources at the State Department, told reporters in a teleconference.

“With the Kerch Strait closure and other incidents, it is hard for anyone to rely on and trust that,” he added. “We do not see the maintaining of gas transit volumes, however modest, as a real proposal.”

Fannon, who made the comment after a tour of eastern Europe, warned that if gas volumes via Ukraine fell it would over time degrade the pipeline infrastructure and harm the continent’s energy security.

Europe, which relies on Russia for over a third of its gas needs, currently gets most of its supplies delivered via Ukraine, which derives up to 3 percent of its gross domestic product from transit charges. (Reporting by Alissa de Carbonnel; Editing by Jan Harvey)