Russian President Vladimir Putin told US President Barack Obama by telephone that Moscow reserves the right to protect its interests and those of Russian speakers in Ukraine. In response, Obama told Putin that Russia had committed a clear violation of Ukrainian sovereignty by sending forces into Crimea and warned of consequences.
"The United States condemns Russia's military intervention into Ukrainian territory," the White House said in a statement outlining what was discussed in a 90-minute phone call between Obama and Putin.
The White House said Obama called on Russia to de-escalate tensions by pulling its forces back and refraining from further interference in the Ukraine and noted that the United States will suspend participation in preparatory meetings for G8 summit in Sochi, Russia.
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In a statement posted online, the Kremlin said Obama had expressed concern about the possibility of Russian military intervention in Ukraine.
"In response to the concern shown by Obama about the plans for the possible use of Russia's armed forces on the territory of Ukraine, Putin drew attention to the provocative, criminal actions by ultra-nationalists, in essence encouraged by the current authorities in Kiev," the statement said.
"The Russian President underlined that there are real threats to the life and health of Russian citizens and compatriots on Ukrainian territory. Vladimir Putin stressed that if violence spread further in the eastern regions of Ukraine and in Crimea, Russia reserves the right to protect its interests and those of Russian speakers living there."
US Defense Secretary Chuck Hagel told his Russian counterpart on Saturday that Moscow's military intervention risked creating further instability and an escalation "that would threaten European and international security," the Pentagon said.
"Secretary Hagel stressed that, without a change on the ground, Russia risks further instability in the region, isolation in the international community and an escalation that would threaten European and international security," Pentagon spokesman Rear Admiral John Kirby said in a statement, describing the call with Russian Defense Minister Sergei Shoigu.
When a convoy of Russian military vehicles unloaded dozens of armed troops into this sleepy Crimean port town on Saturday, residents thronged around them honking car horns, snapping pictures and waving Russian flags.
Although the Russian-speaking servicemen bore no insignia, their vehicles had Russian military plates and there was no doubt among residents they were deployed from the nearby Russian base to take up position outside a Ukrainian border guard base.
Ludmila Marchenko, a retired teacher, simply burst into applause when asked about the masked soldiers with automatic rifles standing guard nearby.
"At first we were in shock, now we see it as a liberation," the 66-year-old told Reuters.
Russian President Vladimir Putin demanded and won approval from his parliament on Saturday for military action in Ukraine to protect Russian citizens. Moscow says it has not yet decided whether to send troops. But clearly it has already acted here.
Those residents who felt foreboding as they watched the armored vehicles roll mostly hung back in the crowd.
"This is a mess. This is an invasion. I think this is an act of aggression by Russia," said Dmitry Bessonov, 55, a retired miner from Donetsk.
Such voices may be in the minority. Ethnic Russians, who are in the majority in the Crimean peninsula, have been angered by nationalist rhetoric from the protesters who toppled pro-Russian President Viktor Yanukovich after months of demonstrations in Kiev's main square, known as the Maidan.
"They made a big mistake when they stood on Maidan and said they wanted to ban the Russian language ... We don't want to be second-class citizens," said Marchenko's brother Vitaly, a civilian sailor.
"I am not against a united Ukraine ... Yes, our president was not great. Yes, there was corruption and theft, but we don't want to live under these conditions. We are just sick of these speeches by fascists and neo-fascists."
The sight of Russian boots on the ground here on the outskirts of Sevastopol - home to the Black Sea Fleet – is nothing unusual to residents, who quickly adapted to the presence of more than 100 armed men parked along the main strip of the bay that is popular with tourists.
The masked soldiers barked at reporters to "move back", refusing to identify themselves, but stood congenially shoulder -to-shoulder with residents who posed for photographs. They happily accepted cigarettes from the crowd.
In a bizarre carnival-like scene, Russian Orthodox priests chanted prayers, while a wedding party drove by loudly honking their car horns.
"It is a great joy for us," said Vladimir Tikhonov, 53, an electrician. "I want this to be Russian land and it will be."
Valentina Magomedova, an accountant whose curiosity drew her to the scene, said people regretted a decision by Soviet leader Nikita Khrushchev, himself a Ukrainian, to transfer the Crimea from Russia to Soviet Ukraine in 1954.
"The new authorities (in Kiev) are not legitimate. We trust Putin, we love Russia," she said. "We were part of Russia and we are sorry still that Khrushchev gave us away."
While most residents had no love for Kiev's new leaders, some were worried by the dangers of the situation and wary of Russia's designs. Confronting the mute soldiers, one man vented his frustration, "What are you doing here? Get lost."
"I have a business, tourist season is beginning, I can't have a war," he said under his breath, shaking his head and turning his back on the 10 trucks and five armoured vehicles.
A nearby restaurant decided to shut its doors early and keep them shut for the next few days.
"No one asked us. We are like puppets for them ... We have one Tsar and god – Putin," she quipped.