Loud bangs
A loud bang was heard about 23:10 local time (21:10 GMT), following by a smaller blast, BBC reporters in Simferopol say.
A city resident blogged about hearing an explosion come from the direction of Elevatornaya Street, where an electronic combat military unit is based.
Nato: "Call on Russia to withdraw its forces to its bases"
There are also unconfirmed reports, on social media, that Russian forces are trying to take over another Ukrainian military base, this time at Sevastopol's military airport, Belbek.