Initial reports said the truck had smashed through the gates and that post in the city of Sevastopol was being stormed but the reporter could not see any signs of the gates being damaged.
A Ukrainian military official, Vladislav Seleznyov, said by telephone that the armed men took over the base without any shooting and that no one was hurt.
Another Ukrainian official told Reuters at the post that he was now mediating between the Ukrainian forces and the armed group inside, and that no arms had been seized.