The U.S. intends to deploy 10 interceptor missiles in Poland and a radar in the Czech Republic citing a threat from Iran and other "rogue states," while Russia views these plans as a destabilizing factor for Europe and a threat to its national security.
"An issue about how Russia might fit and cooperate and be a part of those efforts, that probably is for Sochi or later - it may not get done by Sochi. There's no deadline here," National Security Advisor Steve Hadley told reporters on board Air Force One during the U.S. president's flight to Ukraine on Monday.
George W. Bush arrived in Ukraine late on Monday to start a week-long tour of Eastern Europe, of which the highlight will be a NATO summit in Bucharest, Romania.
He will meet with Russia's President Vladimir Putin over the weekend in the Black Sea resort of Sochi for the last time before Putin leaves his post.
"Sochi is for an opportunity for these two leaders to get together again. It is an opportunity to reach an agreement on missile defense. But, hey, if we don't have it by Sochi, we'll keep working it," Hadley said.
Recently, there has been some progress in the long-running dispute between the two countries over the missile defense issue.
During their visit to Moscow in late March, U.S. Defense Secretary Robert Gates and Secretary of State Condoleezza Rice offered to let Russia monitor the proposed U.S. missile and radar bases in Eastern Europe.
Washington has also offered not to activate the system until there is a "clear and present" threat from Iran or other potential adversaries.
"I think we're moving in a direction...where Russia and the United States could have missile defense as an area of strategic cooperation," Hadley told reporters.
Russia's Foreign Minister Sergei Lavrov praised last week U.S. moves to ease Russian missile shield concerns, but said giving up the idea of opening new missile bases in Central Europe was the best way of addressing Moscow's unease.