Western nations have called on Russia to ease tensions in Ukraine's Crimea region after armed men seized the local parliament and raised the Russian flag.
Russia also scrambled fighter jets along its borders as part of military exercises it announced a day earlier.
Moscow said it was willing to work with the West on averting a crisis, but warned foreign powers against taking decisions on behalf of Ukrainians.
Meanwhile, the ousted Ukrainian president is reported to be in Russia.
Viktor Yanukovych plans to hold a news conference in the southern Russian city of Rostov-on-Don on Friday, Russia's Ria news agency reports.
Earlier, in his first statement since being voted out of office by MPs last week, Mr Yanukovych said he had been "compelled to ask the Russian Federation to ensure my personal security from the actions of extremists" and that he still considered himself the legitimate president of Ukraine.
Also on Thursday, Ukraine's new interim government - including Prime Minister Arseniy Yatsenyuk - was approved by parliament.
Unidentified armed men entered the Crimean parliament in the regional capital Simferopol by force on Thursday morning, and hoisted a Russian flag on the roof.
They were cheered by a handful of pro-Russian demonstrators who gathered round the building, despite a police cordon.
"We've been waiting for this moment for 20 years," the protest leader said. "We want a united Russia."
The men are believed to be still in the building, although it is not clear if they have made any demands or statements.
They did put up a sign reading "Crimea is Russia" and threw a flash grenade in response to questions from a journalist, AP news agency reported.
Western leaders were quick to urge Moscow and Crimean activists not to escalate tensions further.
Nato Secretary General Anders Fogh Rasmussen said he was "concerned about developments in Crimea" and urged Russia "not to take any action that can escalate tension".
US Defence Secretary Chuck Hagel told Russia "not to take any steps that could be misinterpreted, or lead to miscalculation, during a very delicate time".
Later US Secretary of State John Kerry said Russia had reaffirmed it would respect Ukraine's territorial integrity - but the US would look for action to back up the statements.
Both UK British Prime Minister David Cameron and German Chancellor Angela Merkel voiced their concerns at a joint news conference in London.
Earlier, Ukrainian interim President Olexander Turchynov warned Russia that any movement of its Black Sea Fleet beyond its base in Crimea would be seen as "military aggression".
The warnings came as Russia performed a second day of military exercises, saying its fighter jets were on "combat alert".
"Constant air patrols are being carried out by fighter jets in the border regions," Russia's defence ministry told Interfax news agency.
Russian President Vladimir Putin ordered a snap drill on Wednesday to test the combat readiness of troops in central and western Russia, near the border with Ukraine.
Russian Foreign Minister Sergei Lavrov has stressed the need to implement an EU-brokered peace deal agreed between Mr Yanukovych and opposition parties before his departure from office last week.
But John Kerry said later Mr Yanukovych had left the post of president - the US vice-president had tried for about 10-12 hours to get in touch with him after he fled Kiev with no success.
Amid the rising tensions, the Crimean parliament announced it would hold a referendum on expanding the region's autonomy on 25 May.
The uncertainty in Ukraine has sent its currency, the hryvnia, tumbling to a record low.
New PM Arseniy Yatsenyuk accused Mr Yanukovych and his government of stripping the state coffers bare, telling parliament billions of dollars had been transferred to offshore accounts in the past three years.
The International Monetary Fund said it had received a request for assistance from the new government and would be sending a team to Kiev in the coming days.
Tensions have been rising in Crimea since Mr Yanukovych was ousted.
On Wednesday the city saw clashes erupt between Ukrainians who support the change of government and pro-Russians.
Crimea - where ethnic Russians are in a majority - was transferred from Russia to Ukraine in 1954.
Ethnic Ukrainians loyal to Kiev and Muslim Tatars - whose animus towards Russia stretches back to Stalin's deportations during World War Two - have formed an alliance to oppose any move back towards Moscow.
Russia, along with the US, UK and France, pledged to uphold the territorial integrity of Ukraine in a memorandum signed in 1994.