Britain's defense ministry says Russian President Vladimir Putin is likely to declare the annexation of some regions in Ukraine on Friday after announcing the results of so-called "referendums" that have been held there.

Pro-Russian forces began holding the votes last Friday in the eastern and southern regions under Russian control. They said voting will continue until 4 p.m. on Tuesday.

Russia's state-run Tass news agency said that voter turnout exceeded 50 percent in the regions of Donetsk, Luhansk, Zaporizhzhia and Kherson.

The leader of pro-Russian militants in Luhansk, Leonid Pasechnik, said the referendum is now valid and that preliminary results could be released on Tuesday night.

Russian media are reporting that annexation procedures could be taken after approval by the Russian parliament.

Britain's defense ministry said on Tuesday that Putin is scheduled to address Russia's parliament on Friday. The ministry said Putin may formally declare the accession of the occupied regions of Ukraine to the Russian Federation.

The ministry said Russia's leaders almost certainly hope an accession announcement will be seen as a vindication of what Russia calls its special military operation and "consolidate patriotic support for the conflict."

But it said the aspiration "will likely be undermined by the increasing domestic awareness of Russia's recent battlefield set-backs and significant unease about the partial mobilization announced last week."