Ukraine crisis: US urges restraint and warns it is 'watching Russia'
The US has called for all sides to "step back and avoid any kind of provocations" amid heightened tensions in Ukraine's Crimea region.
Secretary of State John Kerry said he had spoken to his Russian counterpart who promised to respect Ukraine's "territorial integrity".
But he warned Moscow needed to back up its words with actions.
Earlier, pro-Russian armed men stormed Crimea's local parliament, while Russia has been conducting military exercises.
The ousted Ukrainian President Viktor Yanukovych - on the run since he was voted out of office last week - also reportedly surfaced in Russia.
Ukrainian media said he arrived in the southern Russian city of Rostov-on-Don early on Friday, where he is due to give a news conference.
In statement on Thursday he said he still considered himself the legitimate president.
A new interim government - including Prime Minister Arseniy Yatsenyuk - was approved by parliament on Thursday.
US Vice President Joe Biden pledged his country's "full support" with future reforms in Ukraine, in a telephone call to Mr Yatsenyuk.
Words 'not enough'
Mr Kerry sought assurances from Russian Foreign Minister Sergei Lavrov in a phone call on Thursday, asking Russia "to support Ukraine to rebuild unity, security and a healthy economy".
Mr Lavrov, he said, relayed a message from President Vladimir Putin that Russia would respect the territorial integrity of Ukraine and that snap military drills ordered on Wednesday were unrelated to the events in Ukraine.
But Mr Kerry said words were not enough. "We will look to Russia for the choices that it makes in the next days for their confirmation of these statements," he said, in a joint news conference with German Foreign Minister Frank-Walter Steinmeier.
Russia, Nato and some Western leaders have expressed concern at the unfolding events in Crimea.
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".
Russia scrambled jets to monitor its borders on Thursday.
Also on Thursday, Mr Lavrov confirmed Russia would work with the West but warned foreign powers against taking decisions on behalf of Ukrainians.
He stressed the need to implement an EU-brokered peace deal agreed between Mr Yanukovych and opposition parties before his departure from office last week.
The uncertainty in Ukraine has sent its currency, the hryvnia, tumbling to a record low.
Mr Yatsenyuk has 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 traditionally Russian-leaning Crimea since Mr Yanukovych was ousted.
Early on Friday, a group of armed men in military dress reportedly descended on the airport in the regional capital of Simferopol, Interfax-Ukraine news agency says, quoting eyewitnesses.
The news website Ukrayinska Pravda reports that there were four Kamaz trucks and around 150 armed men, some speaking with "thick Russian accents".
However the airport was reportedly working as normal. One unnamed source at the airport told Russia's Ria Novosti news agency that the report it had been seized "does not correspond to reality".
In a separate incident on Thursday, unidentified armed men entered Crimea's parliament building by force 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.
On Wednesday the city saw clashes erupt between Ukrainians who support the change of government and pro-Russians.
Amid the rising tensions, the Crimean parliament announced it would hold a referendum on expanding the region's autonomy on 25 May.
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.
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