Russia sent fighter jets to patrol the border with Ukraine today, as the interim president warned any movements of its troops from their Crimea naval base will be considered an act of "military aggression".
Meanwhile, a Russian news organisation said the country was providing shelter for ousted president Viktor Yanukovych in a Kremlin sanatorium just outside Moscow.
Ukrainian interim President Olexander Turchynov condemned the takeover of government and parliament buildings in Crimea as a "crime against the government of Ukraine" following a seizure by pro-Russian activists in the early hours of this morning.
He warned the military will use "all methods necessary" to protect citizens shortly before the Ukrainian parliament approved the formation of a national coalition government.
He said: "Unidentified people with automatic weapons, explosives and grenades have taken over the governmental buildings and the Parliament building in the autonomous region of Crimea.
"I have given orders to the military to use all methods necessary to protect the citizens, punish the criminals, and to free the buildings."
His warning came as Arseny Yatsenyuk, who was approved as Ukraine's Prime Minister today, said the government would have to take "unpopular" decisions as the country "is on the brink of economic and political collapse".
In pictures: Crisis in Ukraine
In pictures: Crisis in Ukraine
1/11 Demonstrations in Crimea
Crimean Tatars drag away a police officer in front of a local government building in Simferopol
2/11 Demonstrations in Crimea
Pro-Russian activists pray outside the Crimean Parliament building in Simferopol
3/11 Demonstrations in Crimea
Crimean Tatars hold flags during rallies near the Crimean parliament building in Simferopol
4/11 Demonstrations in Kiev
Young girls chant the song 'Glory to Ukraine' at Independence Square in central Kiev
5/11 Demonstrations in Crimea
A man receives medical treatment after he was injured in clashes during rallies near the Crimean parliament building
6/11 Demonstrations in Crimea
Pro-Ukrainian activists hold placards reading "Crimea +Ukraine is love" during a rally in front of the Crimean parliament in Semfiropol
7/11 Demonstrations in Kiev
Protestors capture a military armoured vehicle in central Kiev
8/11 Demonstrations in Kiev
An old man stands with an Ukrainian flag on Kiev's Independence Square
9/11 Demonstrations in Kiev
A man lays flowers at one of the barricades heading to Kiev's Independence Square
10/11 Demonstrations in Kiev
Flowers cover the ground and barricades where protesters were killed in a recent clash with riot police in Kiev's Independence Square
11/11 Demonstrations in Kiev
A self-defense unit patrolling the city centre in Kiev, Ukraine
The 39-year-old has previously served as the economy minister and foreign minister, and a speaker of parliament.
In a speech to parliament after his nomination, Mr Yatsenyuk charted a course towards the European Union and urged Russia to co-operate with Ukraine following Mr Yanukovych's removal from office.
"There is no other way out for us than to take extremely unpopular decisions," he said to loud applause.
Crimea "has been and will be a part of Ukraine" but the country would not accept the secession of the southern Crimea region.
The International Monetary Fund (IMF) has confirmed it will dispatch a fact-finding team to Ukraine in the coming days in response to its request for support.
The IMF managing director, Christine Lagarde, said they were discussing how to help Ukraine with its international partners after Ukraine's new finance minister, Oleksandr Shlapak, requested they send a mission.
"It will be a new programme. We will be asking for at least $15bn and then it will become clear," he was quoted as saying by the Interfax news agency.
Earlier, Mr Yanukovych allegedly asked for Russia's protection from "extremists" and declared himself still the legitimate leader of the country. The Ukrainian fugitive president is seeking refuge in a luxury health resort outside Moscow, the Russian news agency RBC has claimed.
Mr Yanukovych said many parts of Ukraine had been enveloped by an "orgy of extremism" and he and his closest aides had been threatened physically.
"I have to ask the Russian authorities to provide me with personal safety from the actions of extremists."
Russian news agencies say he will hold a news conference in southern Russia tomorrow.
Mr Yanukovych was ousted on Saturday after three months of unrest led by protesters in Kiev. He has since fled the area and is wanted by authorities over the violence that led to the deaths of more than 80 people during three days of bloody clashes.
A spokesman for Mr Putin said they were not aware of Mr Yanukovych's reported arrival in Moscow.
Additional reporting by agencies