The US military says it struck dozens of targets across Iran, including air defence systems, radar sites, missile and drone facilities, and small boats.
Iranian media reported explosions in Bandar Abbas, Qeshm, Sirik, Jask and parts of Khuzestan.
One person was killed and four wounded in a strike on a water pumping station in Mahshahr.
Iran says it retaliated with missile and drone attacks on US-linked military sites in Jordan, Bahrain and Kuwait.
Jordan says it intercepted four missiles, while Kuwait says its air defences engaged hostile aerial targets.
Oil prices rose, with Brent crude climbing above $79 a barrel.