The United States has issued a fresh warning against Russian military intervention in Ukraine after it emerged troops were on alert for an urgent drill to test their readiness for combat.
Secretary of State John Kerry said any action would be a "grave mistake" - a remark that was set to fuel already-heightened Russian suspicions over Western intentions.
He said it would be hypocritical for Moscow to send troops into another country after spending the last several years opposing foreign military action in the likes of Libya and Syria.
"Any kind of military intervention that would violate the sovereign territorial integrity of Ukraine would be a huge, a grave mistake," he said.
Mr Kerry, who also announced that the Obama administration was planning $1bn in loan guarantees for Ukraine and would consider additional direct assistance in the future, insisted that US policy toward Ukraine was not aimed at reducing Russia's influence.
Meanwhile, Ukraine's ex-president Viktor Yanukovych - who has been put on the international wanted list - remains in the country, according to the deputy general prosecutor.
Mykola Golomcha said: "We have information indicating Yanukovych is still in Ukraine."
He did not give further details of the whereabouts of the leader who has been missing since Friday when he fled Kiev. He is wanted by police in the country on charges of mass murder.
Ukraine's pro-European protest leader Arseniy Yatsenyuk has now been nominated as premier until presidential elections are held in May.
The appointment came amid rising tensions in Crimea. Pro-Russia separatists and supporters of Ukraine's new leaders came head to head outside Crimea's regional parliament before a key debate.
Around 2,000 people, many of them ethnic Tatars who are the indigenous group on the Black Sea peninsula, gathered in Sevastopol in support of the 'Euro-Maidan' movement.
In Kiev, several hundred pro-Russia demonstrators chanted their loyalty to Moscow and denounced the "bandits" who had seized power in the Ukrainian capital.
Isolated skirmishes could be seen among the crowds and there were reports of missiles being hurled, but police appeared to be holding the two sides apart as the emergency session took place inside the parliament to discuss the crisis.
Crimea was gifted to Ukraine in 1954 in the Soviet-era by then Soviet leader Nikita Khrushchev. With a part of Russia's Black Sea fleet based in the port of Sevastopol, it remains the only region of Ukraine where ethnic Russians dominate in numbers.
Russia's foreign minister said the "nationalist and neo-fascist" sentiment in western Ukraine must be "decisively condemned".
Sergei Lavrov's warning came as Ukraine's acting interior minister said he had disbanded the elite Berkut riot police that protesters blamed for scores of deaths in last week's clashes.