MOSCOW — Russia seized two American diplomatic properties on Friday and ordered the United States Embassy in Moscow to reduce its staff by September, in the Kremlin’s first retaliatory steps against proposed new American sanctions.

The moves, which Russia had been threatening for weeks, came a day after the United States Senate approved a measure expanding economic sanctions against Russia, as well as against Iran and North Korea. The bill, mirroring one passed by the House on Tuesday, now goes to President Trump for his signature.

It is unclear whether Mr. Trump will sign. Given the congressional investigations into possible collusion between his campaign and the Kremlin, and considering that the Republican Party has majorities in the House and the Senate, he is under considerable pressure not to veto the sanctions.

But the White House has been ambivalent about whether Mr. Trump will give his approval. During his campaign for the presidency, he pledged to improve ties with Russia.

The United States Embassy in Moscow confirmed in a statement that it had received the notification from the Russian Foreign Ministry and was sending it to Washington for review.

The American ambassador, John F. Tefft, expressed “his strong disappointment and protest” at Russia’s actions, the statement said.

The statement from the Russian Foreign Ministry said that the United States Embassy was asked to reduce its diplomatic and technical staff members in Russia to 455 by Sept. 1, matching the number of Russian diplomats in the United States.

In addition to the main embassy in Moscow, the United States maintains consulates in St. Petersburg, Yekaterinburg and Vladivostok.

It was not immediately clear how many American workers would have to leave, because the Kremlin’s announcement did not detail which employees were to be included in the count. There are hundreds of staff members in Russia, including workers constructing an embassy building in Moscow.

Starting on Aug. 1, Russia will also block access to a warehouse in Moscow, as well as to a bucolic site along the Moscow River where staff members walk their dogs and hold barbecues.

In December, President Barack Obama expelled 35 Russian diplomats and seized two estates, one on Long Island, N.Y., and one on Chesapeake Bay in Maryland, in response to Russia’s meddling in the United States presidential election.

Moscow did not respond at that time, with President Vladimir V. Putin signaling that he was hoping for better relations under President Trump. Those hopes have largely evaporated. On Thursday, Mr. Putin said at a news conference in Finland that he would wait to see the final law on the sanctions before deciding on a response. But the Senate vote tipped the balance.

The announcement from the Foreign Ministry said that if the United States responded to the latest measure with any further expulsions, Russia would match them.