US President Barack Obama has arrived in Estonia for talks on Russia and the Ukraine crisis with Baltic leaders.
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".
The rapid-response force and other security measures will be discussed at a two-day Nato summit in Wales, which begins on Thursday.
Mr Obama arrived in Tallinn early on Wednesday. He will later meet Estonian President Toomas Hendrik Ilves, Lithuania's Dalia Grybauskaite and Latvia's Andris Berzins.
The BBC's Gavin Hewitt says the three former Soviet states have been unsettled by President Vladimir Putin insistence that Russia has a right to intervene to defend the interests of Russian speakers.
The White House said Mr Obama would use his trip to Estonia, where about 25% are ethnic Russians, to make it clear that it is "not OK for large countries to flagrantly violate the territorial integrity of their smaller neighbours".
(BBC News)
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