Russian Foreign Ministry spokesperson Maria Zakharova (file photo by AP)

Russia says Ukraine has deployed half of its army, or 125,000 troops, to the ethnic Russian region of Donbass, amid an escalation between Kiev and separatists in the volatile zone.

Russian Foreign Ministry spokeswoman Maria Zakharova made the remarks during a briefing in Moscow on Wednesday.

Earlier in the day, the Kremlin spokesman Dmitry Peskov said Moscow was seriously worried about the large number of Ukrainian troops near Russia. He also accused Kiev of trying to take back, by force, the eastern Donbass region, which has been the scene of fighting between the Ukrainian army and pro-Russia forces. He said de-escalation was impossible at the moment, because half of Ukrainian armed forces had been deployed along the Russian border.

Russia cannot take any measures to de-escalate, Peskov said. We have seen the messages that more than 120,000 (Ukrainian) troops have been deployed to the conflict zone. This raises our fears that sentiments (in Ukraine) in favor of a military solution to the Donbass problem could prevail.

This is very dangerous adventurism. All this causes us deep concern because this is happening in the immediate vicinity of our borders.

President Vladimir Putin has also expressed concern about the presence of NATO troops near Russian borders.

The Russian president says he wants serious talks with Washington and its allies in order to obtain guarantees that NATO will not expand further East. He has accused the West of breaking its promises regarding its military presence in the region.

Ukraine has denied the accusation, with President Volodymyr Zelensky calling for talks with his Russian counterpart to resolve the crisis in the east of his country. But the Kremlin says the conflict is a domestic Ukrainian issue, and has nothing to do with Russia. The conflict has killed over 13,000 people since 2014.

Russia orders US diplomats to leave

In a separate development on Wednesday, the tit-for-tat spat between Russia and the United States deepened after Moscow ordered some American diplomatic staff to leave the country in retaliation for Washington’s decision to expel over two dozen Russian diplomats.

Elsewhere in the briefing, Zakharova said the US embassy staff who have been in Moscow for more than three years were being ordered to fly home by January 31. She also warned that another group of US diplomats will be forced to leave by July 1 if Washington fails to reach a compromise and waives the three-year staying rule for Russian diplomats. We ... intend to respond in the corresponding way.

The Russian official said the US has no authority to determine the terms of departure and suspension of Russian staffers.

Washington has ordered 27 Russian diplomats and their families to leave by the end of January. The US claims its move is not to expel but not to renew visas for those diplomats intending to stay beyond three years.

Ties between the US and Russia have been strained over a host of issues, including Ukraine.