Russian troops have seized control of a Crimean naval base at Feodosia, the third such attack in 48 hours, Ukrainian officials have told the BBC.
Defence spokesman Vladislav Seleznyov said the Russians had attacked the base from two directions using armoured personnel carriers and stun grenades.
Russia has taken over most of Ukraine's military bases in Crimea.
The G7 group of industrialised countries is to consider a collective response to the crisis in The Hague.
Leaders of the G7 will hold talks on Monday on the sidelines of a long-planned summit on global threats to nuclear security.
Russian President Vladimir Putin is staying away from the nuclear conference, choosing to send his foreign minister instead.
A Ukrainian soldier there told Reuters news agency that shots had been fired and confirmed that the base had been taken over.
The BBC's Mark Lowen in Crimea said Russian troops were now in complete control and the Ukrainian soldiers had been taken away.
Feodosia was one of the last remaining bases under Kiev's control, but had been surrounded by Russian forces for some time, says our correspondent in Crimea's capital Simferopol.
Two other military bases were stormed and seized on Friday.
Russian defence officials said earlier that the tricolour of Russia had been hoisted at 189 Ukrainian military units and facilities in Crimea.
The view in Kiev is one of deepening concern, says the BBC's David Stern in the Ukrainian capital.
The question now, he says, is how Ukraine and the West will respond, and what Russia's next move will be.
Nato's military commander in Europe warned on Sunday that Russian forces on Ukraine's eastern borders were capable of mounting an operation all the way to Moldova.
Russia annexed Crimea following a Moscow-backed referendum in the area on 16 March.
Moscow's move came after protesters overthrew pro-Kremlin President Viktor Yanukovych in February.
The Kremlin said it had acted to protect its "compatriots" in Crimea from "fascists" moving in from the mainland Ukraine.
The US and EU have responded with a series of sanctions targeting those individuals including senior officials that they accuse of involvement in Crimea's annexation.