Richard Nephew, the deputy special envoy for Iran, left his post as part of the U.S. team negotiating Iran's return to the 2015 Joint Comprehensive Plan of Action (JCPOA).

A State Department official confirmed to Reuters on Monday that Nephew was no longer part of the U.S.'s negotiating team, but he will remain a State Department employee. The official did not provide a reason for Nephew's resignation but noted that such a move was "very common" after a year in the administration.

The Hill has reached out to the State Department for further comment.

Earlier on Monday, The Wall Street Journal reported that Nephew had resigned due to a difference of opinion on how to approach the negotiations with Iran, reportedly advocating for a tougher posture in negotiations. The U.S. has yet to directly engage in negotiations, participating through European allies.

Administration officials told the Journal that two other members of the negotiating team had decided to step back from engaging in talks due to sharing in Nephew's opinion of having a tougher approach.

As Reuters noted, news of these internal disagreements within the U.S.'s negotiating team comes at a critical point in the talks with Iran, with the U.S. and its European allies recently saying there are only weeks left to salvage the agreement.

The eighth round of negotiations began last month, with Western diplomats warning that this was the last chance for Iran to offer serious resolutions in rejoining the JCPOA.

On Monday, it was reported that Iran had rejected a precondition set by the U.S. of releasing American prisoners. U.S. diplomats have said that it is unlikely that the JCPOA can be revived without the release of these prisoners. While Tehran has signaled that it is open to conducting a prisoner exchange, it has rejected the notion of tying this action to the nuclear deal.