MOSCOW, May 30 (Reuters) - U.S. plans to further curb operations of Russia's so-called "shadow fleet" of oil tankers are unacceptable, Russian Foreign Ministry spokesperson Maria Zakharova said on Thursday.

The United States and its allies are weighing additional sanctions and could act further to increase Russia's cost of using a shadow fleet of tankers to evade a Group of Seven countries' cap on the Russian oil price. (Reporting by Dmitry Antonov; writing by Vladimir Soldatkin; editing by Jason Neely)