The United States might delay activating its proposed missile defense sites in Europe until it has "definitive proof" of a missile threat from Iran, US Defense Secretary Robert Gates said Tuesday. At a news conference after meeting Czech Prime Minister Mirek Topolanek, Gates said this was a proposal to the Russians - who strongly oppose US 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.