Pro-Russian activists have taken control of the HQ of Ukraine's navy in the Crimean city of Sevastopol. Ukrainian navy chief Serhiy Hayduk has been detained and the Russian flag is flying over the compound. Many Ukrainian personnel have left the base. On Tuesday, Crimean leaders signed a treaty with Moscow absorbing the peninsula into Russia. That followed Sunday's referendum approving Crimea's split from Ukraine - a vote that has been widely condemned.