Russia said on Wednesday it was ordering US embassy staff who have been in Moscow for more than three years to fly home by Jan 31, a retaliatory move for what Moscow said was a US decision to limit the terms of Russian diplomats.

The step, the latest in an escalating diplomatic row, comes after Russia's ambassador to the United States said last week that 27 Russian diplomats and their families were being expelled from the United States and would leave on Jan 30.

"We ... intend to respond in the corresponding way. US embassy employees who have been in Moscow for more than three years must leave Russia by Jan 31," Foreign Ministry spokesperson Maria Zakharova told a briefing.

The RIA news agency cited her as saying that new US rules meant Russian diplomats who had been forced to leave the United States were also banned from working as diplomats in the United States for three years.

"Before July 1 next year, unless Washington waives the three-year rule and compromises, more (US) workers (in Russia) will leave in numbers commensurate with the number of Russians announced by the State Department," she said.

There was no immediate comment from the US embassy in Moscow.

Further reductions in US embassy staff in Moscow would put pressure on an operation that Washington has already described as being close to a "caretaker presence" amid tit-for-tat expulsions and other restrictions.

The embassy is the last operational US mission in the country after consulates in Vladivostok and Yekaterinburg were closed and it has shrunk to 120 staff from about 1,200 in early 2017, Washington says.

Deputy Foreign Minister Sergei Ryabkov said it was not too late for Washington to stop Moscow following through on the new expulsions if it abandoned its own plans to force out Russian diplomats.

Ties between Washington and Moscow, at post Cold War lows for years, are under pressure due to a Russian troop build-up near Ukraine.