US Secretary of State Marco Rubio will travel to the UAE, Kuwait and Bahrain this week to discuss Washington's initial deal with Iran and other regional issues, the State Department announced on Monday.
His trip comes after the US and Iran signed a preliminary agreement to end the war and reopen the Strait of Hormuz.
Mr Rubio, a longtime Iran hawk, is also US President Donald Trump's National Security Adviser and will be expected to convince allies of the merits of the interim agreement and any final deal that could follow.
All three countries Mr Rubio is visiting were repeatedly struck by Iranian drones and missiles during the conflict.
He will "meet with officials about a range of US policy priorities including the [agreement] with Iran, efforts to secure full and free safe transit through the Strait of Hormuz, and discussing shared priorities across the region with Gulf Co-operation Council members", State Department spokesman Tommy Pigott said in a statement.
Mr Rubio was standing behind Mr Trump as the President signed the preliminary agreement in Versailles, outside Paris, last week.
But until now, most of the public messages about the Iran deal has been left to Vice President JD Vance, who was in Switzerland for an opening round of negotiations at the weekend. He said talks on Iran's nuclear programme are expected to start this week and reported "good progress".
In Bahrain, Mr Rubio is set to meet the GCC to discuss shared priorities across the region.
GCC leaders have broadly supported efforts to end the Iran conflict, but many are concerned about the terms of the interim deal that Mr Trump signed last week.
One point of particular concern among regional officials is the possibility of a $300 billion reconstruction fund for Tehran that Gulf leaders worry would be used to rebuild its military capacity and to fund regional proxy groups.
The US on Monday issued a general licence for Iran, which authorises the production, delivery and sale of crude oil as well as petrochemical and petroleum products of Iranian origin until August 21. That six-month period could easily be extended or made permanent, if talks progress.
The interim deal's failure to address Iran's ballistic missile programme is also concerning for Washington's Gulf allies.
Myriad details remain to be decided over the next two months of talks, but when Mr Vance was pressed on the issue of ballistic missiles last week, he said every country has a right to defend itself.
One major sticking point in negotiations is the situation in Lebanon and the extent to which Iran is prepared to persuade Hezbollah to stand down from fighting Israel, which has invaded southern Lebanon and carried out strikes across the country.
Mr Rubio spoke to Israeli Prime Minister Benjamin Netanyahu and Lebanon’s President Joseph Aoun on Friday about solidifying the ceasefire in Lebanon and about future talks.
“As a result of those calls, the US started a monitoring mechanism via Centcom so that our policymakers have real-time and accurate information about fighting in Lebanon,” a US official said in a statement.
Israeli and Lebanese officials will be in Washington for three days this week for a new round of talks.
