WASHINGTON  Now that President Obama has established what he called a “clear timetable” for Iran to halt its nuclear program  progress must be made by the end of the year, he declared on Monday  both American and Israeli officials are beginning to talk about how to accomplish that goal.

But even after Prime Minister Benjamin Netanyahu’s first visit to the White House on Monday, where he and Mr. Obama stressed common goals and played down their differences on strategy, it seemed clear that they still do not agree even on the basic question of how much time remains to stop Iran from gaining a nuclear weapons capability.

So now begins Mr. Obama’s diplomatic sprint. His declaration Monday that “we’re not going to have talks forever” was a warning to the Iranians that his fundamentally different approach  serious American engagement with Tehran for the first time in three decades  must bear fruit before Iran clears the last technological hurdles to building a weapon. It is a strategy that some administration officials describe as “negotiations with pressure”  a combination of direct negotiations, reassurances that Washington is no longer seeking regime change in Iran, and an effort to persuade the ruling mullahs that the alternative to serious concessions will be painful: international sanctions that are far harsher and more tightly enforced than the weak mix of actions imposed so far by the United Nations Security Council.

“There’s a three-way race going on here,” one of Mr. Obama’s strategists said the other day. “We’re racing to make diplomatic progress. The Iranians are racing to make their nuclear capability a fait accompli. And the Israelis, of course, are racing to come up with a convincing military alternative that could plausibly set back the Iranian program.”