“It’s almost saying to them, put up or shut up,” said an official. “It’s not that the Russians get to say, ‘We’ll try and therefore you have to suspend.’ It says the threat has to go away.”

The issue was expected to be discussed further at a meeting on Friday in Geneva between Foreign Minister Sergey V Lavrov and Secretary of State Hillary Rodham Clinton. Obama and Medvedev will meet for the first time on April 2 in London, officials said.

Obama’s letter, sent in response to one he received from Medvedev shortly after Obama’s inauguration, is, officials in Washington said, part of an effort to “press the reset button” on Russian-American relations.

The plan to build a radar facility in the Czech Republic and deploy 10 interceptor missiles in Poland — a part of the world Russia once considered its sphere of influence — was a top priority for President Bush to deter Iran in case it developed a nuclear warhead to fit atop its long-range missiles. Bush never accepted a Moscow proposal to install part of the missile system on its territory and jointly operate it so it could not be used against Russia.