On Tuesday, the agency named Poland, the Czech Republic and Britain as possible candidates to help bolster a fledgling US missile-defence shield against any missiles shot from the Middle East.
The initial configuration of the ground-based leg of the system, put together by Boeing, is aimed at thwarting ballistic missiles fired from North Korea.
Henry Obering, the air force lieutenant-general, told reporters after a speech to an annual missile-defence conference in Alabama, on Tuesday: "We will have a recommendation with respect to sites and alternative sites" in Europe in the next several months.
But he said he did not know when the so-called third site would be chosen.
The site would join Vandenberg air force base in California and Fort Greely, Alaska, as home to silos housing US interceptor missiles.
Obering said the United States planned to install 10 ground-based interceptors at the European site by the end of 2011. By then, up to a total of 43 interceptors are due to have been installed in Alaska and California.
Asked to describe the selection criteria, he spoke of optimising protection for both the United States and its European allies from any Middle Eastern warheads. In addition, he cited the positioning of radar systems that would support the new site, infrastructure considerations and soil make-up.
Obering said that the Pentagon's missile defence agency has begun working with Israel to help find ways to counter enemy rockets, a much shorter-range threat than the "Star Wars" mission to block ballistic missiles for which it is known.
Asked about the missile threat from Iran, Obering said: "We're concerned about them. That's one of the reasons why we have designed our system to be able to handle threats from that region."