At a news conference after meeting Czech Prime Minister Mirek Topolanek, Gates said this was a proposal to the Russians - who strongly oppose U.S. missile defense - that has yet to be worked out in detail.
"We would consider tying together activation of the sites in Poland and the Czech Republic with definitive proof of the threat - in other words, Iranian missile testing and so on," Gates said with Topolanek at his side.
The United States wants to build a missile interceptor base in Poland and a radar site in the Czech Republic, but details have yet to be negotiated.
"We have not fully developed this proposal, but the idea was we would go forward with the negotiations, we would complete the negotiations, we would develop the sites, build the sites, but perhaps delay activating them until there was concrete proof of the threat from Iran," the defense chief said.
U.S. officials have said that the proposal tying activation of the European sites to proof of an Iranian threat was presented to the Russians by Gates and Secretary of State Condoleezza Rice earlier this month. But Gates' remarks in Prague were the most specific and clear that such a proposition raises the prospect of delay.
Iranian officials insist their nuclear effort is entirely peaceful - designed to produce energy only - but the Bush administration and some of its allies suspect Tehran of covering up a clandestine weapons program.
Much of the disagreement between Washington and Moscow over missile defense in Europe has centered on the question of when Iran's missile program would reach the stage where it could threaten all of Europe and the United States. The Russians say that is a far-distant prospect; the Americans say it is coming soon.
Gates described a related proposal to the Russians that might mean permitting a Russian presence at U.S. missile defense bases, including at the Polish and Czech sites. He said this was presented to the Russians in the interest of making as transparent as possible to Moscow how the missile defense sites operate.
Asked whether having Russians on his territory would be acceptable to the Czech government, Topolanek pointedly declined to say. "No comment," he said through an interpreter. Prior to the breakup of the Soviet empire, Czechoslovakia was part of the Warsaw Pact that opposed the U.S.-led NATO alliance.
Gates stressed that any proposal that involved allowing a Russian presence on Czech soil as monitors or inspectors of the radar site would be presented first to the Czech government and would not be negotiated with the Russians unless the Czechs agreed. Topolanek nodded his head.
Meanwhile, Iran's new top nuclear negotiator was to hold his first meeting with the EU's foreign policy chief over Tehran's contentious nuclear program on Tuesday in Rome.
Saeed Jalili, a little known diplomat, was appointed his country's chief negotiator after Ali Larijani stepped down this weekend. The departure of the moderate Larijani was seen as a victory of hardline President Mahmoud Ahmadinejad that could push the Islamic Republic into an even more defiant position in its standoff with the West.
However, Larijani was set to attend the Rome talks Tuesday alongside Jalili and the EU's Javier Solana, according to Iran's Foreign Ministry. The talks had been scheduled before Larijani's replacement.
On Tuesday, officials in Solana's office declined to confirm the meeting's agenda, saying only that the discussions aim to push Tehran to enter formal negotiations on its nuclear program.
The U.N. Security Council has imposed two sets of sanctions over Iran's refusal to suspend uranium enrichment.
The U.S., Russia, China, Britain, France and Germany, with E.U. support, agreed last month to delay until November any new U.N. resolution to toughen sanctions, giving Iran more time to cooperate with an investigation into past nuclear activities by the U.N.'s nuclear watchdog, the International Atomic Energy Agency.
Larijani was viewed as more moderate compared with Ahmadinejad, and the two often clashed over how to negotiate with the world on the nuclear issue.