Secretary of State Antony Blinken said Tuesday that "hundreds" of US sanctions will remain on Iran even if the United States rejoins a nuclear accord.

President Joe Biden's administration has been engaged in indirect talks with Iran about reversing former president Donald Trump's exit from the 2015 nuclear accord, formally known as the Joint Comprehensive Plan of Action (JCPOA).

"I would anticipate that, even in the event of a return to compliance with the JCPOA, hundreds of sanctions remain in place, including sanctions imposed by the Trump administration," Blinken told a Senate hearing.

"If they are not inconsistent with the JCPOA, they will remain unless and until Iran's behavior changes," he said.

The discussions in Vienna, brokered by European diplomats, have been locked in dispute on which sanctions to lift.

The Biden administration is ready to end the sweeping measures imposed by Trump -- including an effort to stop all of Iran's oil exports -- if it reverses the steps away from the nuclear deal that it took to protest the last administration's sanctions.

But Iran has insisted on a removal of all sanctions -- while the Biden administration has insisted that some will remain if they were imposed over other concerns, including human rights and Iran's support for extremist movements.

Blinken reiterated support for returning to the nuclear accord, with which UN inspectors said Iran was complying before Trump pulled out the United States.

Asked about concerns that Iran did not declare all activities from before the nuclear deal, Blinken said: "Plain and simple, we would be in an even better place to insist on it answering those questions if we had managed to get Iran back into compliance with the JCPOA and if we were part of it, too."

"But regardless, it needs to answer those questions. It needs to come clean about past activities," Blinken said.