"(It) looks like to me that the ingredients are coming together where that could be a distinct possibility," Bush said Thursday during a news conference with NATO Secretary-General Jaap de Hoop Scheffer.
"NATO could assure its members and the people within NATO that there would be defenses available to prevent a Middle Eastern nation, for example, from launching a strike which could harm our security," Bush said.
Scheffer said NATO would "take a clear position on missile defense, recognizing the threat and working on the answers to that recognized threat."
Russia has voiced security concerns if a defense system were installed.
Bush, meeting with Romanian President Traian Basescu, said he would stress "Russia is not our enemy" when he meets with Russian President Vladimir Putin.
"(He) needs to understand the missile defense system is aimed at primarily at rogue regimes coming out of the Middle East that could hold us all hostage," Bush said.
"Terrorism is a threat equally for Russia, for America, for Spain, and it can materialize at any time against Ukraine, against Georgia," Basescu said. "(We) are all at the same level of risk."