The US cleared the way for $6 billion in oil proceeds to be returned to Iran and agreed to release five Iranians as part of a secretive but widely expected deal that will pave the way for several jailed Americans to return home.

Secretary of State Antony Blinken notified Congress on Monday of a waiver that will let German, Irish, Qatari, South Korean and Swiss banks transfer the $6 billion from South Korea without fear of running afoul of US sanctions. He said the $6 billion would be held in restricted accounts in Qatar, where it will be “available only for humanitarian trade,” according to a copy of the notification, which was obtained by Bloomberg News.

The letter to Congress didn’t say when the prisoner exchange would take place. The State Department didn’t immediately respond to a request for comment. The Associated Press reported the notification earlier Monday.

US officials had announced the broad outlines of the deal in early August after Iran moved four US citizens from prison to house arrest. The American prisoners include Siamak Namazi, who has been held in Tehran’s Evin prison since October 2015.

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At the time, US officials declined to describe details of the deal, saying that revealing more risked upsetting a delicate process that could still fall apart. They were also wary of acknowledging talks with a regime that has ramped up human rights abuses and continues to supply weapons and other material to Russia for its invasion of Ukraine.

Yet people familiar with the matter have said that the US and Iran have been engaged for months in tentative and secretive diplomacy that’s seen the two sides inch toward an informal understanding under which Tehran would free the Americans and potentially impose limits on its nuclear program.

It’s all part of a broader effort by the administration to restore at least some of the restrictions Iran agreed to under the terms of a 2015 nuclear deal, the Joint Comprehensive Plan of Action. Former President Donald Trump quit that agreement in 2018.

Republicans and some Democrats in Congress have been critical of the Biden administration’s bid for new diplomacy with Iran, saying it will only encourage the regime to jail more Americans and press ahead with its nuclear development. Iran denies that it has any plan to acquire a nuclear weapon.

(Updates with details of agreement starting in second paragraph)