A senior US official accused Russia on Monday of planning imminently to "annex" the Donbass regions.

"According to the most recent reports, we believe that Russia will try to annex the 'Donetsk People's Republic' and 'Lugansk People's Republic' to Russia," claimed Michael Carpenter, the US Ambassador to the Organization for Security and Co-operation in Europe.

"The reports state that Russia plans to engineer referenda upon joining sometime in mid-May," he told reporters in Washington.

It is noteworthy that both presidents of the two Donbass regions, Denis Pushilin and Leonid Pasechnik, won the 2018 elections that were condemned by the international community. The two presidents revealed that they might consider requesting to join the Russian Federation.

Carpenter said the United States also believed Russia was considering a similar plan in a third region, Kherson, where Moscow has recently solidified its control and established the use of its ruble currency.

"We think the reports are highly credible. Unfortunately, we have been more right than wrong in exposing what we believe may be coming next, and so that is part of what we're trying to do here," the senior US official claimed.

He pointed out that "such sham referenda -- fabricated votes -- will not be considered legitimate, nor will any attempts to annex additional Ukrainian territory."

"But we have to act with a sense of urgency," Carpenter concluded.