"All of the leaders agreed on the need for Russia to pull its military forces back to their bases, allow for the deployment of international observers and human rights monitors to the Crimean peninsula, and agree quickly to de-escalate the situation and restore Ukraine's soverignty and territorial integrity, a White House statement issued after the calls said.
The president phoned the leaders separately from Key Largo, Fla., where he is spending the weekend.
"The leaders rejected the proposed referendum in Crimea as a violation of Ukraine's constitution and underscored that all decisions about the future of Ukraine must include the government in Kyin," the statement said. "The leaders made clear that Russia's continued violation of international law will isolate it from the international community."
Russia's state-supported news agency RIA Novosti said warning shots were fired at a mission from the Organization for Security and Co-operation in Europe Saturday near a checkpoint in the city of Armyansk and the military assessment team was denied entry to Crimea.
No one was injured.
It was the third time the 54-member OSCE team -- which was invited by the central government in Kiev to visit the country, has tried to enter largely pro-Russian Crimea since Thursday.
Russian Foreign Minister Sergei Lavrov Saturday blasted Ukraine's interim government and its effort to vilify Russia's intervention in Crimea.
Lavrov's comments came at a press conference in Moscow, RIA Novosti reported.
"The so-called 'interim government' [in Ukraine] is not independent and, sadly, depends on radical nationalists who seized power in an armed coup," Lavrov said.
Ukraine's uprising was led by Right Sector, a confederation of far-right ultranationalist groups. The organization has no seats in the newly formed Cabinet, but has conducted armed patrols of Kiev, leading to at least one physical altercation, he said.
"The so-called 'Right Sector' is calling the shots through terror and intimidation," Lavrov added.
Lavrov also said Russia is willing to work with the international community to find a solution to the situation in Ukraine, but that other countries should stop demonizing Russia.
"We should have a fair dialogue like partners, without attempts to paint Russia as almost a party to the conflict," Lavrov said.
Russian lawmakers Friday said they would support a vote in the Crimea region to break away from Ukraine and join the Russian Federation, despite threats of sanctions and warnings that the vote would violate Ukrainian and international law, the New York Times reported.
"We will respect the historic choice of the people of Crimea," said Russian lower house speaker Sergei Y. Naryshkin.
However, Russian President Vladimir V. Putin said earlier this week that he did not foresee Crimea becoming part of Russia.
Notable deaths of 2014 [PHOTOS]