Ukraine crisis: Obama in Estonia ahead of Nato summit - V?DEO
He is due to hold talks in the capital Tallinn with the presidents of Estonia, Latvia and Lithuania.
Correspondents say the three states, which joined Nato in 2004, are worried about Russia`s intervention in Ukraine.
Later in the week Mr Obama will attend a Nato summit that is expected to back plans for a rapid-response force that could be dispatched within 48 hours.
Nato recently announced plans to set up the force to protect Eastern European members against possible Russian aggression.
Russia reacted by saying it would alter its military doctrine to reflect "Nato infrastructure getting closer to Russian borders".