It was claimed earlier this week more than 100 Scots had signed up to fight against Russia for the Ukrainian military.
Russian Defence Ministry spokesman Igor Konashenkov said all foreign mercenaries, detained in Ukraine, would be brought to justice on criminal charges.
"I wish to make an official statement that none of the mercenaries the West is sending to Ukraine to fight for the nationalist regime in Kyiv can be considered as combatants in accordance with international humanitarian law or enjoy the status of prisoners of war," Mr Konashenkov said, according to Russia's Tass news agency.
He said "at best, they can expect to be prosecuted as criminals" and warned foreign nationals "to think a dozen times" before travelling to Ukraine to take part in the conflict.
Ukrainian president Voloymyr Zelensky has said he wants to create an international legion to help defend his country and appealed to foreign volunteers to come forward.
He said on Thursday in a video address around 16,000 volunteer troops had been assembled from countries around the world.
The Ukrainian consulate in Edinburgh said this week their phones had been "ringing off the hook" with offers.
Both the UK and Scottish governments have warned against anyone other than trained military personnel heading to the country.
A Scottish Government spokesperson said: “The Scottish Government understands the strength of feeling of those wishing to support the people of Ukraine following the Russian invasion.
“The best way to help the people of Ukraine is to donate funds to, or volunteer with, charities that are registered in Scotland, or the rest of the UK, that have an ongoing relief operation. They can also donate to the Disaster Emergency Committee appeal at www.dec.org.uk.”
The spokesperson added: “We are advising people to follow Foreign Office travel advice, which is not to travel to Ukraine. Those currently in Ukraine are advised to leave as soon as they can.”