President Barack Obama has arrived in Estonia to make a symbolic show of solidarity with Eastern Europe in the face of a Russian-backed military offensive in Ukraine.
Mr Obama is meeting Baltic leaders to offer reassurances to Nato member states near the Russian border.
The visit comes ahead of a high-stakes Nato summit that begins tomorrow in Wales. Mr Obama and Western allies at the summit will approve plans to position at least 4,000 troops and military equipment in Eastern Europe.
Ukraine is not a Nato member but members of the alliance in Eastern and Central Europe fear being targeted by Russia.
Mr Obama will hold a news conference with Estonian president Toomas Hendrik Ilves, then broader security talks that include the leaders of Latvia and Lithuania.
White House officials say Mr Obama will reassure the Baltics that the US would come to their defence it they were attacked. Under the Nato charter, an attack on one member is considered an attack on the entire alliance.
During the Nato summit in Wales, the alliance will also agree on a more robust rapid response force that will involve positioning more troops and equipment in the Baltics and elsewhere in Eastern Europe. It is unclear whether the plan will satisfy the concerns of the Baltic nations, who have been pressing Nato for permanent bases in the region.
Even before the Ukraine crisis, relations between the Baltic countries and Russia were chilly. Moscow routinely accuses them of discriminating against their Russian-speaking minorities.
About a third of Estonia's 1.3 million residents have Russian as their mother tongue. Many of them feel detached from Estonian society and get their news from Kremlin-controlled Russian TV stations.
The Baltics were invaded by the Soviet Union and Nazi Germany during World War II. After the Soviet Union crumbled, the Baltic countries turned to the West and joined the European Union and Nato in 2004, much to the chagrin of Russia.
Mr Obama will depart Tallinn later today for Wales, which is hosting the two-day Nato summit.
Meanwhile, a spokesman for the Russian president said Vladimir Putin has discussed a peace settlement in eastern Ukraine with President Petro Poroshenko and found that they "largely share views" on ways out of the crisis.
Mr Putin and Mr Poroshenko met last week for talks which mainly highlighted the vastly conflicting views of the two leaders.
Ukraine and the West have accused Russia of sending its troops and weapons to support pro-Russian insurgents who have been fighting government troops in eastern Ukraine since mid-April. Moscow has vehemently denied this charge.
Mr Putin's spokesman Dmitry Peskov told Russian news agencies that Mr Putin and Mr Poroshenko found that they "largely share views" on what could be done to stop the bloodshed in Ukraine's east.