The United States earlier this month stopped processing visas in Moscow, with Russians obliged to head to the US embassy in Warsaw.

"We need to make progress soon," a senior State Department official told reporters.

"We're going to confront the situation -- not next month, but sometime next year -- where it's just difficult for us to continue with anything other than a caretaker presence at the embassy," he said.

"We will do everything humanly possible to keep that mission open," he said, while warning that more functions of the embassy, such as sending diplomatic cables, would become difficult without more staff.

He said that the United States lacked staff for basic tasks such as opening and closing the embassy gates, ensuring secure telephone calls and operating the elevators.

Russia on August 1 barred embassies from hiring Russian or third-country staff, forcing the United States to lay off more than 200 locals at missions across Russia, according to the State Department.

The United States complains of a lack of reciprocity with Moscow counting local staff in its tally of US diplomats while Washington only factors in Russian nationals in its limit on numbers.

The United States has around 120 people at its Russian missions, far down from 1,200 in 2017, while Russia has some 230 people in the United States, excluding those posted in New York for its UN mission.

"They have a much bigger presence here in the United States than we have in Russia," the official said.

The United States and Russia have tense relations on multiple fronts including US allegations of hacking and election interference by Moscow, although Presidents Joe Biden and Vladimir Putin agreed at a June summit in Geneva to work toward more stable ties.