Barack Obama has claimed Russia is breaking international rules by intruding in Crimea, shortly after Russian President Vladimir Putin defended Moscow's military advance into Ukraine.
With tension between Moscow and Washington continuing to mount, the US President said that Putin is not "fooling anyone" with his stance on Crimea.
Obama said: "There have been reports that Putin is pausing and reflecting on what's happened. There is a strong belief that Russian action is violating international law.
"Putin seems to have a different set of lawyers, but I don't think that is fooling anyone."
However, Obama expressed optimism over the situation and said tensions could be eased in the coming days.
"We may be able to de-escalate over the next several days and weeks, but it's a serious situation and we're spending a lot of time on it," Obama said.
Obama, who is reviewing all possible options to deal with the crisis, spoke to German Chancellor Angela Merkel for more than an hour on the telephone.
Obama's remarks came after Putin's combative appearance in a press conference in Moscow, defending Russia's military build-up in Ukraine.
Putin insisted Russia does not want to annex Crimea but repeated that Moscow will defend its interests in the region.
The Russian leader told reporters: "Regarding the deployment of troops, the use of armed forces. So far, there is no need for it, but the possibility remains. What can serve as a reason to use the armed forces? Such a measure would certainly be the very last resort.
"We are not going to go to war with the Ukrainian people. But there is the Ukrainian army. If we make this decision, we will make it for the people of Ukraine.
"Ukraine is not only our closest neighbour. It is our fraternal neighbour. Our armed forces are brothers in arms, friends. They know each other personally. I'm sure Ukrainian and Russian military will not be on different sides of the barricades but on the same side."
The barbed exchanges between the two leaders coincided with US Secretary of State John Kerry's brief visit to Kiev, where he pledged a $1bn loan to the interim Ukrainian government to help it crawl out of the deepening economic crisis.