Some Western diplomats had expected Mr. Biden to announce a strategic review of the planned missile defense system as a way to defuse tensions between Washington and Moscow. Although Mr. Biden did not go that far, he did leave room in both the speech  and an interview afterward  for unspecified changes in the plan put forward by the Bush administration.

“We will continue to develop missile defenses to counter a growing Iranian capability, provided the technology is proven and it is cost-effective,” Mr. Biden said during the speech.

Foreign policy experts said that the Obama administration was most likely averse to making any outright concessions on the antimissile system just days after the Kyrgyz announcement, fearing it could be interpreted as a sign of weakness. Mr. Biden, they said, seemed to be balancing the need to appear firm with the administration’s hopes to reverse the several-year slide in American-Russian relations. Russian cooperation is considered important to American attempts to keep Iran and North Korea from continuing with their nuclear programs.

The missile defense plan as it had been envisioned by the Bush administration would place missile interceptors in Poland and a radar system in the Czech Republic. The Russians balked at the placement so close to their border, saying it was proof that the system was meant to combat their nuclear arsenal, rather than a missile threat from Iran as President Bush had said. Mr. Biden did not say in his speech where he expected the system to be based.

In an interview after the speech, Mr. Biden declined to say what changes might be considered. “What I did say in the speech is that we would consult with our European allies as well as consult with the Russians,” he said.