MOSCOW (Reuters) - Moscow denounced the American decision to close three Russian diplomatic facilities in the United States as a “blatantly hostile act” that violated international law and demanded Washington reverse the order on Sunday.

The Russian flag waves in the wind on the rooftop of the Consulate General of Russia in San Francisco, California, U.S., September 2, 2017. REUTERS/Stephen Lam

The United States has ordered the closure of the Russian consulate in San Francisco and two buildings housing trade missions in Washington and New York. It is the latest broadside in a tit-for-tat exchange between the countries that has helped push relations towards a new post-Cold War low.

Russian diplomats were working to vacate the properties over the weekend, including the six-story consulate.

“We treat these developments as a blatantly hostile act, a grave violation by Washington of international law,” the Russian foreign ministry said of the U.S. order, which was made in late August in retaliation for Moscow cutting the United States’ diplomatic presence in Russia.

“We urge the U.S. authorities to come to their senses and immediately return the Russian diplomatic facilities,” the ministry added on its website. “Otherwise the USA will bear total blame for the ongoing degradation of the relations between our countries.”

Maria Zakharova, a spokeswoman for the foreign ministry, said that U.S. authorities had told Moscow that they expected Russia to sell the facilities, TASS state news agency reported.

Relations between the two nations have been badly strained since Moscow’s annexation of Crimea in 2014 and the subsequent separatist conflict in eastern Ukraine, developments which led Washington to impose sanctions on Russia.

U.S. President Donald Trump took office in January, saying he wanted to improve ties. But relations have been damaged by accusations from American intelligence officials that Russia sought to meddle in the presidential election, something Moscow denies.

Trump, himself battling allegations his associates colluded with Russia, grudgingly signed into law last month new sanctions against Moscow that had been drawn up by Congress.

When it became clear those measures would become law, Moscow ordered the United States to cut its diplomatic and technical staff in Russia by more than half, to 455 people.