"We've been engaged in indirect conversations for the last couple of months, and it remains unclear whether Iran is willing and prepared to do what it needs to do to come back into compliance," Blinken told a hearing of the House Foreign Affairs Committee on Monday.
"We're not even at the stage of returning to compliance for compliance," he said.
"We don't know if that's actually going to happen."
Blinken made similar comments in an interview in late May, reports Xinhua news agency.
The US and Iran have held five rounds of indirect negotiations in the Austrian capital of Vienna since April 6 aimed at reviving the nuclear deal, formally known as the Joint Comprehensive Plan of Action (JCPOA).
Blinken's words were met with an immediate response from Iranian Foreign Minister Mohammad Javad Zarif.
In a tweet, Zarif said that "it remains unclear whether @POTUS (President Joe Biden) and @SecBlinken are ready to bury the failed 'maximum pressure' policy of Trump and cease using economic terrorism as bargaining 'leverage'".
The US government under former President Donald Trump withdrew from the JCPOA in May 2018 and unilaterally re-imposed sanctions on Iran.
In response, Iran gradually stopped implementing parts of its JCPOA commitments from May 2019.