US Vice President Joe Biden today warned Russia that the US would "never recognise" its occupation of Crimea.
Speaking on a visit to Kiev, he said: "No nation has the right to simply grab land from another nation.
"No nation has that right and we will never recognise Russia's illegal occupation of Crimea and neither will the world."
Following the press conference, the Pentagon announced that it will send around 600 US troops to Poland and the three Baltic states, in a bid to reassure the NATO allies.
Tensions remain high in eastern Ukraine after at least three died in a day of shooting during what was supposed to be an Easter truce.
In an exclusive interview with NBC News, the Ukrainian Prime Minister warns of the dangers of unchecked Russian expansion and aggression.
Russia, the US and the EU say that all sides have agreed to steps to "de-escalate" the crisis in Ukraine at talks in Geneva.