The Kremlin says its patience with a U.S. plan to return the Russian Embassy's compounds is running out.

President Vladimir Putin's foreign affairs adviser, Yuri Ushakov, said Monday that Russia has demonstrated a remarkable restraint by refraining from a tit-for-tat response to President Barack Obama's decision in December to expel 35 Russian diplomats and shutter Russian compounds in Maryland and on Long Island, New York.

Ushakov says while Russia has shown "unusual flexibility," Moscow's patience "has its limits." He urged Washington to take action to "free Russia from the need to take retaliatory moves," emphasizing that Moscow will feel obliged to respond if the matter isn't settled.

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The Russians have been in regular contact with the U.S. about the two compounds, and are now determined to get them back.

The Trump administration told Russian officials last month that it would consider handing the properties back over, if the Russians were to lift their freeze on construction of a new consulate in St. Petersburg, according to the Washington Post. The Russians implemented the construction freeze in response to U.S. sanctions that were imposed following Russia's invasion of Ukraine in 2014.

However, a senior adviser for Secretary of State Rex Tillerson denied the report. "The U.S. and Russia have reached no agreements," said R.C. Hammond, who noted that the next senior-level meeting between the U.S. and Russia will be this month in St. Petersburg.

Russian ambassador Sergey Kislyak told CBS News in June that the Kremlin was not interested in a U.S. government proposal that would allow the Russians to sell off the properties.

Russia's Ministry of Foreign Affairs even issued an ominous warning tweet demanding the return, saying "If the US doesn't restore diplomatic immunity of Russian property, Russia will reply in kind regarding regarding US property in Russia."

#Zakharova: If the #US doesn’t restore diplomatic immunity of Russian property, Russia will reply in kind regarding US property in #Russia pic.twitter.com/rk4QpItiVU — MFA Russia 🇷🇺 (@mfa_russia) June 8, 2017

Putin and President Trump are to have their first meeting at the sidelines of the G-20 summit, being held in Germany on Friday and Saturday.