Ukraine's navy chief has announced that he has switched allegiance to the pro-Russian authorities of the flashpoint peninsula of Crimea.
The move comes a day after Denis Berezovsky was appointed to the post by interim leader Oleksandr Turchynov.
Mr Berezovsky said in a televised statement that he "swears allegiance to the residents of the Autonomous Republic of Crimea".
He said: "I swear to execute the orders of the (pro-Russia) commander-in-chief of the Autonomous Republic of Crimea,"
The seismic declaration came as the Kiev authorities appeared to be losing control of the Russian-speaking Crimea peninsula.
Crimea has plunged further and further into disarray since the ouster of Kremlin-backed president Viktor Yanukovych last weekend.
Armed men that Kiev believes are backed by Moscow have since seized key government buildings in the region.
They have surrounded Ukrainian military bases, and Russia's parliament yesterday approved the deployment of troops into the country.
It is unclear why and how Mr Berezovsky switched his allegiance.
Crimea's newly appointed pro-Russia prime minister Sergiy Aksyonov, who is not recognised by Kiev, said the announcement was a "historic event" at a joint press conference with the navy chief.
Some Ukrainian commanders have refused to give up weapons and bases were surrounded.
Meanwhile, US Secretary of State John Kerry will visit Kiev on Tuesday.
The move is to lend support to Ukraine's new interim leaders adding that Russian forces now controlled the Crimean peninsula.
He will meet senior representatives of Ukraine's new government, leaders of the Rada and members of civil society.
Mr Kerry "will reaffirm the United States' strong support for Ukrainian sovereignty, independence, territorial integrity."
He will also stress the right of the Ukrainian people to determine their own future, without outside interference or provocation.
Earlier, Ukraine's security council ordered the general staff to immediately put all armed forces on highest alert, the council's secretary Andriy Parubiy announced.
The Defence Ministry was ordered to conduct the call-up, potentially of all men up to 40 in a country that still has universal male conscription.
Protests in Ireland
The Association of Ukrainians in the Republic of Ireland said 200 people took part in a demonstration at the Russian Embassy in Rathgar, Dublin.
A second protest also took part at European Union House on Dawson Street.
Tánaiste and Minister for Foreign Affairs Eamon Gilmore will travel to Brussels tomorrow for an emergency meeting of EU Foreign ministers to discuss the situation in Ukraine