SIMFEROPOL, Ukraine (AFP) - The new Ukrainian government will hold its first cabinet meeting on Saturday, amid fears of secession in restive Crimea where pro-Russian forces are tightening their grip despite US warnings.
Heavily armed troops in uniforms with no national insignia took up positions around government buildings and the airport in Simferopol on Friday, as Ukrainian officials accused Russia of "naked aggression".
In Washington, a US defence official said Moscow was thought to have already sent "several hundred" more troops into the Russian-speaking region, where it already has a major military base.
President Barack Obama, speaking in Washington, said: "We are now deeply concerned by reports of military movements taken by the Russian Federation inside of Ukraine."
He added that any violation of Ukraine's sovereignty and territorial integrity would be "deeply destabilising", warning that there "will be costs for any military intervention in Ukraine".
Another senior US official later suggested those costs could include a decision by Mr Obama and top European leaders to skip the summit of G8 industrialised nations in the Black Sea Olympic resort of Sochi in June.
Other possible trade and commercial concessions that the Kremlin has been seeking as recently as this week could also be at risk, the official said.
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.
The country's new interim prime minister Arseniy Yatsenyuk was due to preside over his hastily formed cabinet's first meeting on Saturday.
His 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 International Monetary Fund (IMF) chief Christine Lagarde said there was no need to panic, playing down reports that Ukraine is in urgent need of up to US$15 billion (S$19 million) to maintain government payments.
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".
US envoy Samantha Power went further, calling for an urgent, international mission to "de-escalate the situation" in Crimea and mediate dialogue among all Ukrainian parties.
Close US ally Britain also said that any newly deployed Russian troops not answering to the Ukrainian government should withdraw.
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.
Mr 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.
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, Mr Sergiy Kunitsyn, told local television that 13 Russian cargo planes carrying nearly 2,000 troops had landed at the base by Friday night.
Mr Yanukovych, meanwhile, defiantly surfaced in Russia after a week in hiding, and bitterly attacked the new authorities in Kiev.
The ousted leader 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.
Mr Yanukovych also revealed he had spoken by phone with - but had not met - Mr Putin and expressed surprise that his ally had not yet spoken out on Ukraine since his flight.
The Kremlin issued a brief statement after Mr Yanukovych's comments, saying Mr 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 Mr Yanukovych rejected a deal to open the door to eventual EU membership in favour of closer ties with Russia.
Switzerland meanwhile said it was freezing the assets of 20 Ukrainian figures - including Mr 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.