US aircraft struck Iranian rocket launchers on Larak island in the Strait of Hormuz, a narrow waterway that much of the world's oil ships through.
Iran attacked Kuwait on Thursday, the fifth US-aligned country it has hit since the US bombed Iranian sites on Tuesday.
Kuwait says it is intercepting Iranian drones and explosions have been heard in Bahrain, drawing two more Gulf states directly into a conflict that until now centred on Israel, Iran and US forces. US strikes continued for a ninth straight night and Brent crude broke $90.
US strikes on Iran extended overnight to bridges, an airport and a railway station, Iranian media said, as Kuwait reported damage to an electrical and desalination plant and Cathay Cargo postponed Riyadh freighter services.
Six US soldiers died in a drone strike on Kuwait's Port Shuaiba as the United States bombed targets inside Iran and Tehran closed the Strait of Hormuz again — a conflict now grounding Gulf aviation and rattling global markets.
Iran answered a second wave of US strikes by firing missiles at Bahrain, Qatar, Kuwait and Jordan — all hosts to American forces — while shipping through the Strait of Hormuz halted and Israel warned of an Iranian plot against Trump.