The United States says it has “highly credible” intelligence reports that Russia is planning to annex two Ukrainian regions and declare a third “people’s republic” in the southeast.

Moscow may annex the two existing proxy states – the self-described people’s republics in Luhansk and Donetsk “some time in mid-May,” the U.S. ambassador to the Organization for Security and Cooperation in Europe (OSCE), Michael Carpenter, said on May 2.

There were also indications that Russia had designs on Kherson by turning the southeastern city into a third “people’s republic.” Russia currently controls Kherson. Last week, Moscow said it plans to introduce the rouble as the official currency in the city.

“According to the most recent reports, we believe that Russia will try to annex the ‘Donetsk People’s Republic’ and ‘Luhansk People’s Republic’ to Russia,” Carpenter said.

In 2014, Russia annexed the Crimean Peninsula from Ukraine and backed separatists in the two statelets – known as DPR and LNR – in the east.

At the time, referendums on status were held in those areas, which the international community said were illegal. Days before launching its full-scale invasion of Ukraine, Moscow broke with a years-long policy to recognize the DPR and the LNR as independent.

Speaking to reporters at the US State Department, Carpenter accused Russia of planning to hold a “sham referendum” sometime in mid-May to seize more Ukrainian territory.

“The international community, including the OSCE, where I work as ambassador, has been very clear that such sham referenda … will not be considered legitimate nor will any attempts to annex additional Ukrainian territory.”

Meanwhile, Western and US diplomats believe that Russian President Vladimir Putin could formally declare war on Ukraine as soon as May 9, which would pave the way for the full mobilization of Russia’s reserve forces as Moscow eyes conquering the eastern and southern Ukraine.

Known as Russia’s “Victory Day,” May 9 commemorates Russia’s defeat of the Nazis in 1945.

“I think he [Putin] will try to move from his ‘special operation’,” British Defense Secretary Ben Wallace told LBC Radio last week. “He’s been rolling the pitch, laying the ground for being able to say, ‘Look, this is now a war against Nazis, and what I need is more people. I need more Russian cannon fodder.’”

“[I] would not be surprised, and I don’t have any information about this, that he is probably going to declare on this May Day that ‘we are now at war with the world’s Nazis and we need to mass mobilize the Russian people.’”