"There are some who are making the case publicly that we are in favor of a deal that would just be 10 years in duration, and that is not accurate," said White House spokesman Josh Earnest.
Israel and some domestic critics say a decade-long agreement -- even one which on conclusion would leave Iran at least a year away from building a bomb -- opens the door to a nuclear armed Islamic Republic.
The United States had long urged Iran to dismantle its nuclear programme, but in recent months has shifted focus to convincing Iran to limit enrichment, stay at least a year's work away from developing bomb-grade nuclear material and accept tough international monitoring.
The aim of the talks, Earnest said, was to "not just reach an agreement with the Iranians, but reach an agreement with the Iranians that we can verify on a continuing, ongoing basis."
Iran denies seeking a nuclear weapon and says its enrichment activities are peaceful.