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”.
Pro-Russian rebels have been fighting Ukrainian forces since April. Separatists in the eastern regions of Donetsk and Luhansk declared independence after Russia annexed Crimea from Ukraine.
Since the violence erupted, some 2,600 people have been killed and thousands more wounded.
On Tuesday the UN said the conflict had forced more than a million people from their homes in eastern Ukraine.
Ukraine’s army has seen several losses in recent days after rebels launched offensives in both the Luhansk and Donetsk regions, and further south around the port of Mariupol.
Russia has denied accusations by the West and the Ukrainian government that it is sending troops and military equipment over the border to support the separatists.
Meanwhile, Ukrainian forces have accused the rebels of violating a humanitarian deal reached by the the two sides. They say the separatists killed as many as 100 Ukrainian troops who were evacuating the town of Ilovaysk via an agreed corridor at the weekend.
The rebels deny this. A spokesman told the BBC that the convoy was captured without any fighting because it contained too many soldiers and vehicles.
The BBC’s David Stern in Kiev says information about what happened in Ilovaysk is only beginning to emerge because fierce fighting has made accessing the area difficult.
War in eastern Ukraine: The human cost
- At least 2,593 people killed since mid-April (not including 298 passengers and crew of Malaysian Airlines MH17, shot down in the area) – UN report on 29 August
- 951 civilians killed in Donetsk region alone, official regional authorities said on 20 August
- In some particularly dangerous places, such as Luhansk region, victims are said to have been buried informally, making accurate counts difficult
- Rebels (and some military sources) accuse the government of concealing true numbers
- 260,000 people have fled elsewhere in Ukraine while at least 814,000 have gone to Russia.