Prague, Czech Republic ? The Bush administration has told Moscow it may delay activation of a proposed U.S. missile defense system in Europe until it has “definitive proof” that Iran poses a missile threat, Defense Secretary Robert Gates said Tuesday.

A senior Russian official repeated the Kremlin view that the U.S. misreads Iran’s missile potential. And in Washington, President Bush asserted that a U.S.-linked missile defense system is urgently needed in Europe, illustrating the depth of the divide between former Cold War adversaries.

“We need to take it seriously – now,” Bush said of the possibility Iran will gain the ability to attack Europe.

Bush said intelligence estimates show Iran could have the ability to strike the United States and many European allies by 2015. The Americans say the Russian estimate is 2015 to 2020.

Russia has long opposed U.S. missile defenses, currently limited to a fledgling system based mainly in Alaska, California and Colorado, on grounds that it might undermine the deterrent value of its nuclear arsenal. More broadly, Moscow worries at steps toward closer U.S.-European security ties.

As for the proposal Gates described Tuesday, the Russians have expressed interest but not agreement.

At a news conference after meeting with Czech Prime Minister Mirek Topolanek, Gates said the U.S. would proceed with current plans to build a missile interceptor base in Poland and an associated radar in the Czech Republic, but it might be willing to wait before putting them in working order.

“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 Bush administration has been negotiating with Prague and Warsaw over terms of basing. The Russians have pressed the U.S. to freeze the negotiations, but Gates insisted that won’t happen. By “activating,” Gates meant, in simplest terms, switching on the network of communication links that tie the missile interceptors together with radars for tracking and hitting a ballistic missile in flight.