A Ukrainian naval officer (C) passes by armed men, believed to be Russian servicemen, as he leaves the naval headquarters in Sevastopol, March 19, 2014. Three Russian flags were flying at one of the entrances to Ukraine's naval headquarters in the Crimean port of Sevastopol on Wednesday, suggesting that at least some of the base was under the control of pro-Russian forces. A Reuters witness saw three armed men, possibly Russian soldiers in unmarked uniforms, at the gate where the flags were flying. Also in the area were several unarmed members of so-called "self-defence" units, made up of volunteers who have supported Russia's annexation of Crimea. REUTERS/Vasily Fedosenko (UKRAINE - Tags: POLITICS MILITARY TPX IMAGES OF THE DAY) - RTR3HOP3 Vasily Fedosenko/Reuters
Senior Ukrainian officials planned to travel to Crimea on Wednesday in a bid to avert an escalation in hostilities, a day after two men were killed at a military facility in a gun battle between Ukrainian troops and a militia loyal to the local government.
The prime minister in Crimea, which was nominally incorporated into Russia on Tuesday, said Ukraine's deputy prime minister and defence minister would be turned back, however.
"They are not welcome in Crimea," Sergei Aksyonov was quoted as saying by Interfax news agency. "They will not be allowed to enter in Crimea. They will be sent back."
An Associated Press photographer witnessed several hundred self-defence forces take down the gate and make their way onto the headquarters' premises. They then raised the Russian flag on the square by the headquarters.
The unarmed Crimean self-defence forces waited for an hour on the square before the move to storm the headquarters. Following the arrival of the commander of the Russian Black Sea fleet, the Crimeans took over the building while Ukrainian servicemen did not offer any resistance.
The Associated Press photographer was able to enter the headquarters and saw the Crimean self-defence forces roaming around while the Ukrainian servicemen were packing up and leaving.
Russian President Vladimir Putin on Tuesday signed a treaty to incorporate Crimea into its territory following a referendum in which residents of the strategic region overwhelmingly backed the move.
Jubilant crowds in Moscow and other cities across Russia hailed the annexation while Ukraine's new government called the Russian president a threat to the "civilized world and international security," and the U.S. and Europe threatened tougher sanctions against Moscow.
Russian news agencies on Wednesday quoted Valery Zorkin, chairman of the Russian Constitutional Courts, telling reporters that they have just ruled the treaty to be valid, thus clearing yet another hurdle for Russia to annex Crimea.
The treaty will now only need to be ratified by the Russian parliament.
A Ukrainian serviceman and a member of a local self-defence brigade were killed by gunfire in an incident in Crimea on Tuesday.
Thousands of Russian troops had overtaken Crimea two weeks before Sunday's hastily called referendum, seizing some Ukrainian military bases, blockading others and pressuring Ukrainian soldiers to surrender their arms and leave. Putin insisted the Russian troops were in Crimea under a treaty with Ukraine that allows Russia to have up to 25,000 troops at its Black Sea fleet base in Crimea.
The West and Ukraine described the Crimean referendum as illegitimate and being held at gunpoint.
The United States and the European Union on Monday imposed sanctions on Russia, targeting Russian and Crimean officials with visa bans and asset freezes.
Crimea became a flashpoint of Ukraine's months-long crisis over whether to cast its future with the European Union or Russia. Protests began after President Viktor Yanukovych scrapped a trade deal with the EU in favour of a Russian offer. As protests grew and turned violent, Yanukovych fled to Russia and pro-Western parties installed a new government.