The U.S. told Iran through EU mediators that linking the UN investigation of Iran's undeclared nuclear activity to the reimplementation of the 2015 nuclear deal could delay lifting U.S. sanctions, according to a U.S. official and a think tank expert briefed on the issue.

Why it matters: The issue of the International Atomic Energy Agency (IAEA) investigations is one of the two remaining stumbling blocks in returning to the 2015 nuclear agreement.

• Tehran demands the IAEA probes that focus on uranium particles UN investigators found at several Iranian sites be closed ahead of “reimplementation day” — 120 days after a new nuclear deal is signed.

Driving the news: According to the draft nuclear deal presented by the EU, the agreement's implementation would take place in several stages. The parties would move to the next stage only after full implementation of the steps they committed to taking, sources briefed on the draft said.

• Stage three — reimplementation day — is expected to take place four months after the nuclear deal is signed. In this stage, Iran is expected to finish implementing all of the limitations on its nuclear program and resume the full IAEA inspection regime on its nuclear sites, the sources said.
• The U.S. in return would lift all secondary sanctions on Iran and take other steps, including engaging with the private sector on how to do business with Iran.

Iranian President Ebrahim Raisi said on Monday that resolving the IAEA investigation issue is a pillar of any nuclear deal and without it, “It is meaningless to talk about an agreement."

Between the lines: The four-month period between signing the deal and reimplementation day will give time to reach an agreement between the IAEA and Iran on the open investigations.

• But it is unlikely Iran will agree to give the IAEA the clarifications it seeks. This could eventually lead to a suspension in the implementation of the deal.
• U.S. officials hope the fact that Iran gets most of its sanctions relief only after reimplementation day will give the Iranians an incentive to continue implementing the agreement even if the IAEA investigations are not closed.

What they're saying: "The deal will be step by step and implementation for implementation — the Iranians won’t get it all at once, '' a U.S. official said.

• "We told the Iranians that it is up to them to solve it with the IAEA and there is no reason to link it to reimplementation day. If Iran does that, it risks delaying the lifting of sanctions," the official added.

What to watch: Iran's Supreme National Security Council is reviewing the U.S. response to Iran’s comments on the EU draft nuclear deal.

• Tehran has also demanded guarantees in case of another U.S. withdrawal from the deal in the future. The U.S. official said the Biden administration made it clear to the Iranians that it can’t bind the hands of future administrations.
• The U.S. official said a deal could happen “quickly” but stressed it is also very possible there will be more exchanges of written responses. “This could take some time," the official said.