The Russian Foreign Ministry, saying U.S. officials had broken into residences at Russia's consulate in San Francisco, has threatened retaliation over what it called an illegal act.
Russian staff left the consulate last month, after Washington ordered Moscow to vacate some of its diplomatic properties. The moves were part of a series of tit-for-tat actions during a thorny phase in bilateral relations.
Since then, U.S. officials had occupied administrative parts of the compound, but on Monday they entered residential areas that the departing staff had locked, the ministry said in a statement late Monday.
"Despite our warnings, the U.S. authorities did not listen to reason and did not give up their illegal intentions," it said. "We reserve the right to respond. The principle of reciprocity has always been and remains the cornerstone of diplomacy."
Footage aired repeatedly on Russian state television showed what the broadcaster said were U.S. officials breaking locks that had sealed off parts of the compound and entering the buildings.
The "intruders" had taken over the whole premises, including the consul general's residence, the ministry said.
"Therefore, we understand that Americans, breaking into our diplomatic buildings, have de facto agreed that their missions in Russia may be treated likewise."
A State Department spokeswoman denied Tuesday that U.S. officials had broken into the residences, saying diplomatic security and representatives of the department's Foreign Missions Office had walked through the spaces to ensure they had been vacated by an October 1 deadline.
Spokeswoman Heather Nauert said the United States had "graciously" given the Russian government more time to leave the buildings after discovering they had families and individuals "living in this sort of office-type space."
"Once we learned that, we then offered them extra time there to pack up their items and leave," Nauert said. "So we permitted them living in the apartments until October 1. And their time was up.
"We did not break locks. No FBI [was] involved," she added.
"This is diplomatic security along with the Foreign Missions Office. What they do is they just walk through, look around. The purpose of that is to make sure that people are no longer living there. And they conducted and they completed it."
Expulsions began dispute
In San Francisco, a Department of State Diplomatic Security guard answered the door at the Russian consulate, a six-story brick building surrounded by a modest security fence and manicured hedges in the city's Pacific Heights neighborhood.
The guard declined to comment and left the building in a black SUV.
A sign on the front door said the consulate was closed and gave a new mailing address. The State Department guard left the consulate front door open, behind a closed gate, and sounds of activity could be heard inside, but no one responded to repeated calls by a reporter.
Russian President Vladimir Putin last month accused Washington of "boorish" treatment of Russia's diplomatic premises on U.S. soil, ordering the Foreign Ministry to take legal action over alleged violations of Russia's property rights.
The dispute began late last year when former U.S. President Barack Obama expelled 35 Russian diplomats on accusations of Russian meddling in the election that took Donald Trump to the White House.
Trump took office in January, saying he wanted to improve ties with Russia, while Putin also spoke favorably of Trump.
But the allegations of interference in the vote, which Moscow has denied, have persisted as an investigation by U.S. authorities has widened.
In July, Moscow ordered the United States to cut the number of its diplomatic and technical staff working in Russia by around 60 percent, to 455.