Moscow is threatening to order an additional 155 American diplomatic personnel removed from missions in Russia in a further escalation of the cycle of retaliation between the two world powers.

Russian Foreign Minister Sergei Lavrov said on Monday that Russia was seriously considering the additional cuts, although it had not yet made a formal petition. A senior State Department official, speaking on condition of anonymity, confirmed the Trump administration was aware of the possibility.

Earlier this year, Moscow ordered the U.S. to slash its staff in Russia by nearly two-thirds, to 455 people, by Sept. 1. In response, the U.S. ordered Russia to shutter its consulate in San Francisco and two trade offices, in Washington, D.C., and New York.

Washington and Moscow are basing their tit-for-tat on differing interpretations of “parity,” that principle that each government have an identical numerical presence in the other’s country. That was the justification Moscow used to cut the U.S. staff to 455, the same number Russia had here. The U.S. then said closing the San Francisco consulate meant each country would have three consulates.

Now, Russia said it was being too generous because the number 455 included staff at its mission at the United Nations: 155 people. That number of American employees in Russia may now need to go, Lavrov suggested.

“If they have taken parity as a criterion…we will bring these conditions into full compliance with what is called parity,” Lavrov said at a news conference Monday in Amman, Jordan.

State Department spokeswoman Heather Nauert said the administration continued to hold out hope that relations will improve.

“We don’t want to continue this kind of diplomatic tit-for-tat,” she said. “There are far too many areas where we can, we hope we can, cooperate with Russia.” But asked about further cuts in staff, she said: “I’m not going to get into forecasting any potential Russian reaction.”

The cuts in staffing have crippled U.S. diplomatic functions in Russia, officials say. Visa processing, after a brief suspension, has been renewed but at a much slower pace and only in Moscow, not in the consulates in St. Petersburg, Yekaterinburg or Vladivostok.

The consulates canceled thousands of interview appointments for non-immigrant visa applicants on Sept. 1, and many Russians seeking visas to the U.S. are applying in neighboring countries, such as Ukraine.

In addition, the United States will be forced to turn to contractors for basic security services, the senior State Department official said. The official described the moves as taking a meat cleaver to the diplomatic mission in Russia.

Both governments acknowledge their relations, despite President Trump’s professed admiration for Russian President Vladimir Putin, are severely deteriorated — although the two continue to work on some issues, such as the fight against terrorism, cooperation in Syria and possibly curbing the nuclear threat posed by North Korea.

Secretary of State Rex Tillerson is expected to meet with Lavrov this month on the margins of the U.N. General Assembly in New York, and the State Department’s number-three official, under-secretary for political affairs Thomas Shannon, was in Helsinki, Finland, on Monday to talk with his Russian counterpart, Deputy Foreign Minister Sergei Ryabkov, about the embassies and other issues.

The current downward spiral began in the waning weeks of the Obama presidency, when the last administration ordered the expulsion of 35 Russian intelligence officials and seized two Russian compounds as punishment for the Kremlin’s alleged meddling in the American election process.

Putin did not respond at the time, apparently waiting to see if relations would improve under Trump.

But in July, Congress overwhelmingly passed a new sanctions bill against Russian businesses and individuals, which Trump was forced to sign, albeit reluctantly.

Putin then ordered the first shearing of the American diplomatic corps, from about 1,200 to 455, though more than half of those removed, around 600, were locally hired Russian nationals. Two diplomatic compounds used by the U.S. Embassy staff were also taken back by the Russians, including a storage warehouse in Moscow and a summer cottage dacha in the northern part of the capital.

After Washington ordered closure of the additional offices in the U.S., Russians were further angered at inspections of the offices by U.S. security personnel, refusing an invitation to accompany them on the New York walk-through, although they did attend similar inspections in San Francisco and Washington.

Russian Foreign Ministry spokeswoman Maria Zakharova said the inspections were a “provocation,” while some Russian officials questioned the legality.

“Is it an attempt by the American security services to organize an anti-Russian provocation and, probably, to plant compromising materials into the building and then somehow find them inside?” Zakharova said at a briefing in Moscow.

At Monday’s news conference in Amman, Lavrov said Russia was also considering placing some travel restrictions on U.S. diplomatic staff working in Russia. Currently, American diplomats may officially enter Russia at more entry points than Russians are allowed to use to enter the U.S. Russia might limit those entry points and the number of American diplomatic staff allowed to travel outside of the diplomatic mission zones.

Such restrictions would also be a move toward parity, Lavrov argued: Low- and mid-level Russian diplomats working in the U.S. have only a radius of 25 miles of free movement outside their diplomatic compounds. High-level Russian diplomats are allowed to travel freely.

All U.S. diplomatic staff are allowed to travel freely within Russia, a privilege Lavrov said was now under review.

White House Press Secretary Sarah Huckabee Sanders told reporters last week that Trump had personally approved the decision to take reprisals against Russia. But she added: “We want to halt the downward spiral and we want to move forward towards better relations.”

Special correspondent Ayres reported from Moscow, staff writer Wilkinson from Washington.