The strikes mark the latest escalation of attacks and counterattacks launched by the two sides as they vie for control of the Strait of Hormuz, which carried about a fifth of global oil and gas shipments before the war.
"At 6am ET today, US Central Command forces began launching a wave of strikes against Iran," the US military said on Wednesday.
"The strikes are designed to further degrade military capabilities Iranian forces have used to attack commercial shipping in the Strait of Hormuz."
The US statement gave no further details and there were no immediate reports of attacks in Iranian media.
Late on Tuesday the US military said it had hit dozens of military targets near the Strait of Hormuz and Iranian coastal areas in strikes lasting seven hours.
In response, Iran's Islamic Revolutionary Guard Corps said on Wednesday it had struck US military targets in the region, including in Bahrain, Kuwait and Jordan.
It also threatened on Wednesday to shut off more regional energy exports, saying the US "must brace for the closure of all other export corridors that benefit the US and its allies".
The US has said Iran had attacked seven commercial ships in the past week, leading to nearly a dozen crew members being killed, missing or injured.
The war, which began with US and Israeli strikes against Iran on February 28, triggered Iranian attacks on Gulf states that host US bases and caused major disruption to global energy supplies, raising fears of a surge in inflation.
Oil prices extended gains by about one per cent on Wednesday, after settling on Tuesday on a new one-month high.
An interim ceasefire deal in the conflict signed last month was meant to lead to further negotiations and a permanent truce, but a return to talks has faltered.
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