Pro-Russian forces tightened their grip on airports and strategic sites on Ukraine's Crimea peninsula Friday despite US President Barack Obama warning Moscow not to intervene.
Heavily armed troops in uniforms with no national insignia took up positions around government buildings and the airport in Simferopol, as Ukrainian officials accused Russia of "naked aggression."
In Washington, a US defence official said Moscow is thought to have already sent "several hundred" more troops into the Russian-speaking region, where it already has a major military base.
In a hastily scheduled White House statement with echoes of former Cold War stand-offs, Obama warned his Russian counterpart Vladimir Putin to respect Ukraine's independence.
"The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine," he declared.
The US leader did not elaborate on the nature of these costs but, in an early sign of anger, aides warned that Obama might skip June's G8 summit in Russia's Sochi if the Kremlin's troops enter Ukraine.
Ukraine's interim president Oleksandr Turchynov, who took power after Russian-backed leader Viktor Yanukovych fled last week, directly addressed the Kremlin leader from Kiev.
"I personally appeal to President Putin to immediately stop military provocation and to withdraw from the Autonomous Republic of Crimea... It's a naked aggression against Ukraine," he said.
In New York, the UN Security Council held an emergency meeting to discuss the crisis, but the talks broke up without a formal statement.
Council president Raimonda Murmokaite was able to say only that members expressed support for Ukraine's territorial integrity and sovereignty and for "inclusive political dialogue".
- 'Naked aggression' -
A spokesman for Russia's Crimea-based Black Sea Fleet denied its forces were involved, but uniformed men with assault rifles, body armour and helmets were in evidence around Simferopol.
They sealed off the airport, where civilian flights were halted, and set up barricades to protect Crimea's autonomous government.
Putin this week stoked concerns that Moscow might use its military might to sway the outcome of the standoff by ordering snap combat drills near the border involving 150,000 troops and nearly 900 tanks.
Crimea was attached to Ukraine in 1954 when both Russia and its smaller neighbour were part of the Soviet Union, and most of the population are still ethnic Russians.
Civilian defence groups have sprung up in Crimea since the street uprising in Kiev that last week ousted the pro-Russian president and installed a Western-leaning interim government.
They have torn down some Ukrainian flags, and are angrily preparing for what they fear may be an intervention by extremists from mainly Ukrainian-speaking western Ukraine.
"We're here to prevent fascists or radicals from western Ukraine from coming here by plane," said Vladimir, a 46-year-old former military officer in a group outside Simferopol airport.
Witnesses reported seeing armoured personnel carriers rumble across Simferopol's main streets and at least eight cargo planes bearing the Russian flag landing at a Ukrainian military air base.
A senior official in Crimea, Sergiy Kunitsyn, told local television that 13 Russian cargo planes carrying nearly 2,000 troops had landed at the base by Friday night.
Yanukovych, meanwhile, defiantly surfaced in Russia after a week in hiding, and bitterly attacked the new authorities in Kiev.
The untested new team -- made up in part of leaders of the protests that gripped Kiev for three months -- is grappling not only with growing separatism fears but also the risk of a devastating debt default.
But IMF chief Christine Lagarde said there was no need to panic, playing down reports that Ukraine is in urgent need of up to $15 billion to maintain government payments.
Ukraine's plunging currency rebounded from record lows after Lagarde moved to send a fact-finding mission to Kiev that could open the way for the quick release of about $2.5 billion in EU and US loans.
Yanukovych dramatically ended guesswork about his whereabouts by walking out on stage before 200 reporters in Rostov-on-Don -- a Russian industrial city less than two hours' drive from the border with Ukraine.
The 63-year-old vowed to continue to fight for Ukraine's future but said he would boycott a snap presidential poll that the new Western-backed leadership has set for May 25.
He called the new leadership "young neo-fascists" and blamed the "irresponsible policies" of the West for the escalating crisis.
Yanukovych also revealed he had spoken by phone with -- but had not met -- Putin and expressed surprise that his ally had not yet spoken out on Ukraine since his flight.
The Kremlin issued a brief statement after Yanukovych's comments, saying Putin had stressed in phone talks with EU leaders "the extreme importance of not allowing a further escalation of violence."
Ukraine's bloodiest crisis since independence in 1991 erupted in November when Yanukovych rejected a deal to open the door to eventual EU membership in favour of closer ties with Russia.
- Swiss bank accounts -
Switzerland meanwhile said it was freezing the assets of 20 Ukrainian figures -- including Yanukovych and his multi-millionaire son Oleksandr -- and also launching a money-laundering probe.
Austria and Liechtenstein announced similar moves against Ukrainian figures whom the new authorities say stole billions of dollars in state funds.