Ethiopia's Nobel Peace Prize-winning prime minister is urging calm as millions of citizens hold a referendum on whether to create a new regional state along ethnic lines.

Prime Minister Abiy Ahmed on Wednesday says the Sidama referendum is an expression of Ethiopia's new democratic path.

Abiy's sweeping political reforms since he took office last year have opened the way for some of the country's more than 80 ethnic groups to push for more autonomy.

Sometimes deadly unrest has followed, and tensions could rise ahead of national elections in May. Observers say this poses Abiy's greatest challenge.