"The sense that we have is that, yes, the talks continue to be productive and that progress is being made," said White House spokesman Josh Earnest.
"As long as we are in a position of convening serious talks that are making progress," Earnest said the United States "would not arbitrarily or abruptly end them."
"But if we are in a situation where we sense that the talks have stalled then yes, the United States and the international community is prepared to walk away."
A historic deal is being held up by arguments over the mechanism for lifting crippling sanctions against the Islamic republic.
The United States, Russia, China, Britain, France and Germany want Iran to scale down its nuclear program to extend the "breakout" time needed for it to assemble enough nuclear material to make a bomb.
Earnest said President Barack Obama would make a public address when negotiations conclude, whether successful or not.