The strikes on Iran come on the eve of US midterm elections, and just hours after Trump signalled progress in negotiations with Tehran.
The United States has carried out strikes against Iranian targets in the Strait of Hormuz, a US official said on Monday.
The official, who spoke on condition of anonymity, described the operation as defensive strikes targeting "Iranians using surface-to-air missiles against US asset".
Meanwhile, Iranian media reported five explosions on Qeshm Island in the Strait of Hormuz.
Just hours before, US President Donald Trump signalled progress in negotiations with Tehran.
"Iran wants a settlement, as soon as possible. It could happen!," Trump wrote on his social media platform Truth Social.
Vice President JD Vance told reporters that both he and Trump felt optimistic about "the prospects for potentially coming to a negotiated settlement".
Iranian Foreign Minister Abbas Araghchi said on Monday that indirect talks with the United States were taking place via mediators.
"Given the sensitivity and gravity of the current situation, consultations and deliberations are very intensive," Araghchi told state media.
"Qatar and Pakistan are playing a very active mediating role in these negotiations."
The strikes come on the eve of midterm elections, in which the Republican party is pushing to retain control of Congress.
However, Washington's war in Iran has been a growing point of political contention and a contributing factor to Trump's declining approval ratings, as American's increasingly disapprove of the conflict, particularly over its economic consquences.
Shipping through the Strait of Hormuz, the narrow waterway between Iran and Oman that normally carries around a fifth of the world's oil, has been heavily disrupted since the war began in late February.