President Obama ordered the expulsion of 35 Russian diplomats, closed two rural estates reportedly used by Russian spies, and slapped sanctions on two Russian intelligence organizations and other entities Thursday for their alleged role in what the White House says was a Kremlin-directed effort to interfere with the 2016 presidential race. The sweeping retaliation follows an intense review of what Obama called "aggressive harassment" of U.S. diplomats in Moscow and "cyberoperations aimed at the U.S. election," a hacking campaign that U.S. officials code-named "Grizzly Steppe." It also signaled the worst cyberclash of the modern era, with the two former Cold War adversaries now increasingly focused on penetrating each other's digital networks and communications. In the most dramatic move, the State Department declared 35 intelligence operatives at the Russian Embassy in Washington and the Russian Consulate in San Francisco as personae non gratae. They were given 72 hours to leave the country with their families for "acting in a manner inconsistent with their diplomatic status." The Obama administration also said it would block access after noon Friday to two properties owned by the Russian government — a 45-acre estate along the Chesapeake Bay in Maryland and 14-acre compound in Long Island, N.Y. — that it says were used by Russian personnel for gathering intelligence. The broad penalties, three weeks before Obama hands over the White House to Donald Trump, mark a new low in post-Cold War relations with Russia amid rising tensions over President Vladimir Putin's military operations in Syria and Ukraine. "All Americans should be alarmed by Russia's actions," Obama said in a statement. He said the U.S. moves follow "repeated private and public warnings" to Moscow. "These actions are not the sum total of our response to Russia's aggressive activities," Obama added. "We will continue to take a variety of actions at a time and place of our choosing, some of which will not be publicized." Obama said the Russian effort was aimed at interfering with the U.S. election. He stopped short of endorsing FBI and CIA conclusions that the cyberattack was aimed, at least in part, at helping Trump win. Earlier this month, Obama all but blamed Putin personally, telling reporters that very little happens in the Russian government without Putin's knowledge. Trump has repeatedly dismissed conclusions from the Office of the Directorate of National Intelligence and the Department of Homeland Security that senior Russian officials directed a campaign to interfere in the fall election. In a statement Thursday night,Trump made clear he is still not convinced. "It's time for our country to move on to bigger and better things," he said. "Nevertheless, in the interest of our country and its great people, I will meet with leaders of the intelligence community next week in order to be updated on the facts of this situation." The mass expulsion of alleged Russian spies is the largest in decades, and a Kremlin spokesman, Dmitri Peskov, said Moscow would respond in kind. "I can't say what the response will be, but there is absolutely no alternative to the principle of reciprocity," he said. Peskov said the U.S. measures were "ungrounded and illegal" and were intended to undermine Trump's calls for warmer relations with Moscow.