"We've been engaged in indirect conversations, as you know, 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," Secretary of State Antony Blinken told the House Foreign Affairs Committee.

"We're still testing that proposition," Blinken said.

Former US president Donald Trump pulled out of the Iran nuclear deal in 2018, claiming that it did not do enough to prevent the Islamic republic from building a nuclear weapon.

Trump tightened sanctions on Tehran, and the Iranian authorities responded by loosening restrictions on their nuclear program imposed by the deal.

President Joe Biden has said he would rejoin the agreement if Iran lives up to its end of the bargain.

The two sides have been negotiating in Vienna since April through their partners in the multilateral agreement -- Britain, China, France, Germany and Russia.

The talks are scheduled to resume later this week in the Austrian capital.

"We're not even at the stage of returning to compliance for compliance," Blinken said. "We don't know if that's actually going to happen."