Ukraine interim President Olexander Turchynov warned Russia Thursday not to carry out any “military aggression” by moving troops from its Black Sea Fleet out of their naval base in Sevastopol.
The warning comes after gunmen seized Crimea's regional parliament and the government headquarters of the region, which has a Russian majority population. The men's identity has not been ascertained but they raised the Russian flag over both buildings in Simferopol.
"We ask our Russian partners to provide to stick to their... obligations, we believe Russia would never intervene into Ukrainian domestic affairs and will refrain from any steps that would split Ukraine," Prime Minister designate Arseniy Yatsenyuk told the BBC.
"We are committed to having Ukraine as one united country. We will punish anyone for separatism in Ukraine with all legal and constitutional means," he added.
The warnings from Ukraine's leadership came as Russia carried out a second day of military exercises. "Constant air patrols are being carried out by fighter jets in the border regions," Russia's defense ministry told Interfax.
On Wednesday, President Vladimir Putin ordered a drill to test the combat readiness of troops in central and western Russia, near the border with Ukraine. Thursday's exercises appear to be a continuation of that drill, analysts say.
Pro-Russia separatists and supporters of Ukraine's new leaders came head to head Wednesday outside the regional parliament.
Around 2,000 people, many of them ethnic Tatars who are the indigenous group on the Black Sea peninsula, gathered outside the parliament building in Simferopol on in support of the 'Euro-Maidan' movement which ousted President Viktor Yanukovich.
Meanwhile, several hundred pro-Russia demonstrators chanted their loyalty to Moscow and denounced the "bandits" who had seized power in Ukrainian capital Kiev.
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.
Amid heightened tensions between Russia and the West, 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".
Also on Thursday, former Ukranian President Viktor Yanukovych issued his first statement since being voted out of office by MPs last week, telling Russian news agencies that he still considered himself the legitimate president of Ukraine. He added that he had been "compelled to ask the Russian Federation to ensure my personal security from the actions of extremists."
The state-run Itar-Tass agency quoted an official source as saying Yanukovych would be granted "protection" by Russia, although there was no official confirmation of this.
Svoboda, the ultra-nationalist party, holds several key positions received in the government created by the rebellion in Kiev. This, besides the general prosecutor's office, which it already oversees.