Tensions are escalating in Crimea, one day after Russia annexed the Ukrainian region.
Crimean "self-defense" forces and pro-Russian activists on March 19 broke into the Ukrainian Navy headquarters in Sevastopol.
There were no reports of violence. Ukrainian soldiers were later seen leaving the base accompanied by pro-Russian forces.
A spokesman for Russian forces in Crimea confirmed that the commander of the Ukrainian Navy, Admiral Serhiy Haiduk, has been detained. Haiduk was earlier seen leaving the base and reports said he was then driven away by officers of Russia's Federal Security Service (FSB).
Earlier, the commander of the Russian Black Sea Fleet -- which is based in Sevastopol -- held talks with Ukrainian officers at the naval headquarters.
Elsewhere, a spokesman for Ukraine's Defense Ministry said pro-Russian forces and Russian soldiers used a tractor to ram through the gate of a Ukrainian Navy base in western Crimea. He said they seized the entrance but then halted their advance into the Novoozerne base.
Ukrainian Prime Minister Arseniy Yatsenyuk instructed acting Defense Minister Ihor Tenyukh and First Deputy Prime Minister Vitaliy Yarema to fly to Crimea to "resolve the situation," but Crimea's secessionist leader, Sergei Aksyonov, said they would not be allowed to enter.
WATCH: Pro-Russian forces raise the Russian flag over the naval headquarters.
Meanwhile, Russia's Constitutional Court ruled that the annexation treaty complies with the Russian Constitution.
Russian President Vladimir Putin and Crimea's secessionist leaders signed the treaty on March 18 following a controversial Crimean referendum.
Crimea's Russia-backed authorities said voters overwhelmingly backed the region joining Russia in the referendum on March 16.
The Russian parliament is expected to endorse the treaty as early as this week.
Ukraine and Western states have rejected the vote and denounced the annexation as a "land grab."
The United States and the European Union have imposed travel bans and asset freezes on a number of Russians and Ukrainians over the events in Crimea.
On March 19, Russia accused Western states of violating a pledge to respect Ukraine's sovereignty under a 1994 security agreement.
In a commentary published on its website, the Foreign Ministry said that by "pandering" to a "state coup" in Kyiv, Western countries had violated the Budapest Memorandum.
Under that agreement, the United States, Britain, and Russia guaranteed Ukraine's sovereignty and territorial integrity in exchange for Ukraine's commitment to give up its nuclear arsenal after the collapse of the Soviet Union.
WATCH: Ukrainian troops make camp near Russian border.
Aksyonov said on March 19 that the "patriotic surge" must be "sustained and spread" to other Ukrainian regions with large Russian-speaking populations.
"This patriotic surge we are witnessing today in Crimea and Russia must be sustained and spread to the southeast of Ukraine, where we are being looked upon with hope, where our support and help is awaited," Aksyonov said.
"People really need this today. I am sure that we will sustain this patriotic sentiment toward the people of the southeast."
Putin said on March 18 that Russia did not want -- and did not "need" -- the partition of Ukraine, but that Moscow would protect the interests of Russians.
Two people were reported shot dead in Crimea on March 18.
Ukrainian military officials said a Ukrainian soldier was killed when gunmen attacked a base on the outskirts of Simferopol, the Crimean capital.
A spokeswoman for Crimea's police said a member of the "self-defense" forces had also been killed in the same incident. She blamed both deaths on gunfire from unidentified attackers.
The deaths were the first casualties reported in Crimea since thousands of Russian troops deployed there shortly after pro-Russian Ukrainian President Viktor Yanukovych was ousted in late February.
With reporting by AP, AFP, Reuters, and Interfax