President Bush leaves Monday for a European visit including a NATO summit in Romania and talks about U.S. missile-defense plans with Russian President Vladimir Putin.
Mr. Bush's first stop will be in Ukraine. The president has said he believes NATO should open the membership process for Ukraine and Georgia, but Moscow has made clear it does not want NATO expanding to Russia's borders.
Mr. Putin's successor, President-elect Dmitri Medvedev, says granting NATO membership to the two former Soviet states could threaten European security.
At the NATO summit in Bucharest, alliance leaders are expected to offer membership to Albania, Croatia, and Macedonia. They also plan to talk about expanding the number of troops NATO has in Afghanistan.
The United States and Britain have demanded that their NATO partners make a bigger contribution to the Afghan mission.
Mr. Bush also has accepted President Putin's invitation to visit the Black Sea resort of Sochi later in the week, for talks on the proposed U.S. radar and missile system in Poland and the Czech Republic.
The White House has said Mr. Bush will try to assure the Kremlin leader that the future weapons system is no threat to Russia, but is meant to protect Europe from possible attack by such rogue states as Iran.
Meanwhile, Greece has threatened to block Macedonia's NATO membership bid over the use of the name Macedonia, which is also a region of northern Greece. Athens says the former Yugoslav republic's use of the name implies a claim on Greek territory. Macedonia denies this and says it has met all the criteria for NATO membership.
Some information for this report was provided by AFP, AP and Reuters.