Iran launched missile and drone attacks on US bases in Jordan, Kuwait and Bahrain in retaliation for American strikes on Iranian targets around the Strait of Hormuz.
The strikes came after the US carried out attacks on Iran in response to the downing of an American helicopter, straining a ceasefire that took effect in April.
The exchange drew international calls for restraint on the eve of the World Cup, which the US is co-hosting and Iran is participating in.
It has also cast doubt on US President Donald Trump's claim that negotiations on an enduring settlement to end the Middle East war were in their "final throes."
The conflict, which began with US-Israeli strikes on Iran, threw the region into chaos and rattled global markets before the shaky truce began.
In Bahrain, an AFP correspondent in the capital Manama said several loud explosions could be heard, as Iran's Guards said they had struck a US base there.
Iranian forces also fired "long-range missiles" at US targets in Jordan, the Revolutionary Guards said on Wednesday.
Jordan's military said it shot down five missiles, with no casualties or material damage, while Bahrain said it intercepted and destroyed "a number of Iranian aerial attacks."
Elsewhere, the Kuwaiti military said its air defences were engaging "hostile aerial targets." Iran has recently carried out deadly attacks there too.
Agencies
