A Russian soldier in Crimea opened fire on his colleagues, killing one and wounding another, before turning his weapon on civilians and killing three, an official said on Tuesday.
Three other people were wounded, said Mikhail Razvozhayev, the Russian-installed governor of the Sevastopol region.
He said the attacker had been arrested.
There was no information on the motive for the attacks.
“All circumstances and causes of the incident are being established,” said Razvozhayev, urging people to remain calm and not to spread rumors.
Crimea is a strategic and heavily militarised peninsula on the Black Sea, which Russia seized and annexed from Ukraine in 2014.