JERUSALEM — Israel’s prime minister, Benjamin Netanyahu, moved quickly to block even tentative steps by Iran and the United States to ease tensions and move toward negotiations to end the nuclear crisis, signaling what is likely to be a sustained campaign by Israel to head off any deal.

Netanyahu’s office dismissed as “media spin” a flurry of statements by Iran’s new president, Hasan Rouhani, about the goals of his nation’s nuclear program and his willingness to engage in diplomacy regarding it. His remarks were made amid news that President Obama had reached out to Rouhani with a private letter, and renewed discussion in Washington of talks to lift sanctions against Iran.