The Russian-installed administration of Ukraine's Kherson region said on Monday it was organising some local men into militia units.
In a notice on Telegram, the occupation authorities said men had the "opportunity" to join territorial defence units if they chose to remain in Kherson of their own free will.
However, men in other occupied Ukrainian regions such as Donetsk have previously been compelled to join up and fight with the armies of Russia's proxies in the war with Ukraine.
Russian President Vladimir Putin last week declared martial law in the occupied regions, empowering their Russian-installed administrations to step up mobilisation.
Compelling civilians to serve in the armed forces of an occupying power is defined as a breach of the Geneva Conventions on conduct in war.
Russian authorities have ordered civilians to evacuate from Kherson, one of four Ukrainian regions Russia said it had annexed last month even as Kyiv's forces have made significant military gains.
Russia and its proxies in Kherson have stepped up the urgency of their warnings to leave in the face of Ukraine's counter-offensive.
"It's vital to save your lives," Education Minister Sergei Kravtsov said in a video message on Sunday.
Authorities said at the weekend that around 25,000 people had been evacuated since Tuesday, mainly by boat across the Dnipro River.
(Reporting by Reuters)
