During the call, Obama and Cameroon expressed their grave concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity, the White House said in a statement yesterday.
"Both noted that the current circumstances are unacceptable. Russia has already started to pay a cost for its actions, such as reducing investor confidence in Russia," it said.
"The two leaders welcomed the military observer mission being undertaken by Organisation for Security and Cooperation in Europe at the request of the Government of Ukraine," the statement said adding that they also discussed support for the government of Ukraine as it works to stabilise its economy and makes preparations for elections in May.
The White House said there was a broad international support for the legitimate government of Ukraine; broad condemnation of Russia's actions in violation of Ukraine's sovereignty and territorial integrity.
White House press secretary Jay Carney said efforts were underway both unilaterally and multilaterally to support the new Ukrainian government and to support Ukraine in the difficult time economically; and multilateral and bilateral and unilateral efforts to consider actions to respond to Russia's actions.
Russian President Vladimir Putin, he said, has indicated a pause in some of the activities of Russian military units.
"We are closely monitoring the situation in Crimea and Eastern Ukraine and broadly across Ukraine," he said.
"There is an easy way out for Russia, there is an easy off-ramp, which is to embrace an effort to bring UN or OSCE monitors to Crimea so that they can independently assess the situation on the ground, assess the status of ethnic Russians, and ensure that their rights are being protected -- because we and many others have made clear that it is very important for the Ukrainian government to assure that the rights of all Ukrainian citizens are protected, Carney said.
"But again, there is an easy way out here for Russia, and we certainly hope they take it," Carney said.
Earlier, in a telephone call, US Vice President Joe Biden and Latvian President Andris Berzins expressed their grave concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity and discussed how best to bring about a peaceful resolution.
"The Vice President reaffirmed the United States' longstanding and continuing commitment to peace and security in the Baltic region," the White House said.