“Sometimes these [military] actions — which undermine state sovereignty — could result in a fully-fledged regional war, and even — although I do not want to scare anyone — the use of a nuclear weapon,” Medvedev was quoted by AFP as saying on Thursday during a legal forum in Saint Petersburg.
US Attorney General Eric Holder and some other top international dignitaries and legal scholars also made speeches during the event.
Russia has been at odds with the Western powers over the best way to manage the ongoing violence in Syria, which has led to many deaths over the past year.
Moscow, along with Beijing, has so far vetoed two anti-Syria resolutions, saying a solution to the Syrian crisis must be sought through political dialogue.
Russia was also opposed to the NATO-led war in Libya last year, which has left many civilians dead following the ouster of the country’s former dictator, Muammar Gaddafi.
The former Russian president replaced the country’s newly-elected President Vladimir Putin as Russia’s prime minister on May 7.