US President Obama shows support for Baltic states
3 September 2014 Last updated at 23:47 BST
Russian President Vladimir Putin has said he is hoping for a peace deal between Ukraine and pro-Russian rebels by Friday.
Meanwhile, the US president has urged members of the Nato alliance to send an unmistakable message backing Ukraine - in the face of what he called "brazen aggression" by Russia.
President Obama's comments came as he visited Estonia to try and reassure the Baltic states following recent events in Ukraine, and ahead of a Nato summit in Wales.
Gavin Hewitt reports.