Iran reported new US strikes on Bushehr and other targets as Washington reimposed a naval blockade.
The return to war comes despite a recent memorandum of understanding aimed at ending hostilities.
Attacks on commercial shipping continue, raising global concerns over the Strait of Hormuz and energy security.
Iran reported fresh US strikes on Wednesday on the port city of Bushehr, home to its only civilian nuclear plant, as Washington reimposed a naval blockade in a return to open conflict.
Nearly a month after the US and Iran signed a memorandum of understanding to end the Middle East war that began in February, the two sides resumed fighting with strikes on targets across the region.
Iran's Revolutionary Guards said they targeted the US Fifth Fleet in Bahrain, while Jordan's armed forces said they had downed three missiles from Iran.
US President Donald Trump warned that strikes would intensify next week unless Tehran returns to the negotiating table. "Next week it gets really bad for them," he told Fox News.
At the heart of the resumption of hostilities is the dispute over the Strait of Hormuz, a crucial waterway for global oil and gas flows.
Iran blockaded Hormuz after the US and Israel launched their attack on February 28, using it for leverage before briefly reopening it.
The US has now reimposed its own blockade of Iran's ports. Trump has backed down on a planned 20% levy on ships using the strait, replacing it with trade deals with Gulf allies.
Deputy Foreign Minister Kazem Gharibabadi said the renewed US blockade has dismantled the recent peace understanding.
Five days after the return to war, US Central Command said it struck dozens of Iranian military targets near the strait to degrade Iran's ability to threaten commercial shipping.
Iranian state media reported explosions near Bandar Abbas, Qeshm Island, and Bushehr.
In Kuwait, locals expressed anxiety over the renewed conflict.
Since the war began, Iran has asserted control over the Strait of Hormuz and attacked ships it says are taking unauthorised routes.
Admiral Brad Cooper of CENTCOM said Iran has targeted civilians by attacking seven commercial ships, resulting in nearly a dozen civilian crew members killed, missing, or injured.
Israeli Prime Minister Benjamin Netanyahu warned Iranian leaders that Israel would respond heavily if attacked.
