Russia's central election commission says it will hold elections in September in the four regions in eastern and southern Ukraine that Moscow claims to have annexed, along with the rest of Russia.

Russian President Vladimir Putin unilaterally declared the annexation of the Donetsk, Luhansk, Zaporizhzhia and Kherson regions in September last year.

As part of the annexation, Moscow switched the local currency to the ruble and issued passports for residents, asserting their Russian citizenship.

But Russia has not been in full control of those regions, and fierce fighting is continuing.

Observers say Moscow is holding elections in the areas amid the ongoing major counteroffensive by Ukrainian forces in an attempt to legitimize the annexation.