PRAGUE, Czech Republic — The United States might delay activating its proposed missile defense sites in Europe until it has "definitive proof" of a missile threat from Iran, Defense Secretary Robert Gates said Tuesday.

At a news conference after meeting Czech Prime Minister Mirek Topolanek, Gates said the United States would proceed with current plans to build the sites in Europe but possibly wait before putting them in working order.

The proposal has already been presented to the Russians, who strongly oppose having U.S. missile defense bases in Europe but have expressed interest in the proposal Gates mentioned Tuesday, which Gates said 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.

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. As he said this, Topolanek nodded his head affirmatively.

Earlier Tuesday, Tomas Pojar, deputy minister of foreign affairs, told U.S. reporters traveling with Gates that his government's support for the defense plan is based not only on a shared worry about future missile threats but also a "moral, historical" sense of appreciation for American support for Czech democracy.

He also stressed that Prague does not intend to rush a deal, and he predicted that it will be difficult to win approval in parliament.

"I think it's going to take a few more months" than the U.S. timetable, which calls for completing negotiations by the end of the year and winning parliamentary approval next spring, Pojar said in an interview over breakfast at the Ministry of Foreign Affairs while Gates was meeting with President Vaclav Klaus.

Pojar said he takes little stock in public opinion polls that show a majority of Czechs oppose having a U.S. missile defense site on their territory.

The Pentagon wants to install 10 interceptor rockets in Poland which, when linked to a proposed tracking radar in the Czech Republic and to other elements of the existing U.S. missile defense system based in the United States, could defend all of Europe against a long-range missile fired from the Middle East.

Poland's opposition party ousted ruling conservatives in parliamentary elections on Sunday, which is expected to mean some delay in missile defense negotiations with Washington. Nonetheless, Gates said Monday he still believes Warsaw will cooperate.

Critics say no such system is needed in the foreseeable future because no country in the Middle East, including Iran, now possesses a ballistic missile with sufficient range to threaten all of Europe or the United States.

The U.S. aim is to have both missile defense sites ready for limited operation by 2011 and fully operational by 2013. Russia strongly opposes the U.S. system, including the planned expansion into Europe. Earlier this month, President Vladimir Putin urged Washington to freeze negotiations with Warsaw and Prague.

Many in Congress also oppose adding the two European sites.