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The US military has carried out fresh strikes inside Iran and Tehran has retaliated against an American air base, marking the most serious clash since the two countries’ ceasefire took effect.

Getting into it: A US official said American forces struck a military site overnight on Wednesday and shot down four Iranian one-way attack drones that “posed a threat around the Strait of Hormuz.” The target was an Iranian ground control station in the port city of Bandar Abbas that the US said was about to launch a fifth drone. Officials described the operation as “measured, purely defensive, and intended to maintain the ceasefire,” insisting it was limited and not a return to full-scale combat. It was the second time in three days that the US had hit Iran.

Iran wasted little time hitting back. Early Thursday, its Revolutionary Guard (IRGC) said it had targeted “the U.S. air base identified as the source of the attack,” calling it a “serious warning” and threatening that any further US strikes would draw a “more decisive” response. The IRGC didn’t name the base, and the US didn’t immediately confirm any attack, but the exchange appeared to pull in neighboring Kuwait, whose army said its air defenses were intercepting “hostile missile and drone threats.” Iran and its proxies have gone after that Kuwaiti base before in this war.

The latest round followed Monday’s strikes, in which the US hit Iranian missile sites and boats it said were trying to lay mines in the Strait, prompting Iran to condemn the move as “a grave violation of the ceasefire.”

The clashes have complicated peace talks that just days ago looked close to a breakthrough. Trump said over the weekend that a deal was “largely negotiated,” but by his Wednesday Cabinet meeting he declared the US “not satisfied” and claimed Iran was “negotiating on fumes,” warning, “Maybe we have to go back and finish it, maybe we don’t.”