Iran attacked US military bases in Kuwait on Thursday, despite President Donald Trump threatening further strikes in a fresh flare-up six months into the war.
Trump said his military was prepared to attack Iran "any time we want" after extensive US bombardment of sites across the country left at least 19 Iranians dead.
Tehran retaliated by hitting US military targets across the Middle East, including in Kuwait, Jordan, Bahrain and Iraq's autonomous Kurdistan region.
The clashes deepen the ongoing impasse in the war, as Iran maintains a stranglehold on the Strait of Hormuz oil export route and the US presses its counter-blockade of Iranian ports.
Security chief Mohsen Rezaei warned the United States that Iran had adopted a "new strategy" in the war that "will shatter your foundations".
Kuwait's army said Thursday it was intercepting a missile and drone attack from Iran, with Iranian state TV confirming that Tehran was targeting US bases in the Gulf state "in response to America's crimes against the Iranian people".
The latest round of fighting began when Washington hit Iranian targets on Sunday, the first known strikes in several weeks, saying it was trying to stop Tehran placing sea mines in the Strait of Hormuz.
Iran said Sunday's strikes resulted in "casualties" and responded by targeting US troops in Jordan and the United Arab Emirates.
Projectiles also struck two oil tankers in unclaimed attacks, one of which killed two Filipino sailors, according to a Saudi shipping company.
Agence France-Presse