Ukrainian President Petro Poroshenko says he has instructed all military units near Crimea and in the easterly Donbass region to be at the highest level of combat readiness, following Russian allegations of a Ukrainian incursion into Crimea.
Separately, the spokesman for Ukraine's General Staff told Reuters that Ukraine had been holding scheduled military exercises in southern Ukraine since Wednesday.
Russian President Vladimir Putin accused Kiev on Wednesday of using terrorist tactics to try to provoke a new conflict and destabilise Crimea, which Moscow annexed from Ukraine in 2014.
Ukraine dismissed the allegations as a cynical pretext for Russia to make more military threats against Ukraine.
It was reported earlier that Russia has amassed more troops in recent days equipped with more modern equipment on Ukraine's border with the annexed Crimean region.
"We can unequivocally say that Russian troops who were there since March are now being replaced with others," Oleh Slobodyan, the spokesman for the Ukrainian border guards, said.
"These troops are coming with more modern equipment and there are air assault units. In recent days, we see a strengthening of the units that are at the border. Their number increased," Slobodyan said.
