The United States and Iran traded strikes on Thursday in their most serious clash since an April ceasefire began, rattling ongoing efforts to negotiate an end to the war and open the vital Strait of Hormuz.The latest exchange also drew in US ally Kuwait, which said it was responding to incoming fire, and came as violence on the war's Lebanese front escalated sharply after Israel declared much of the country's south a combat zone.Iranian forces fired at four ships attempting to cross Hormuz, according to state broadcaster IRIB. Iran has blockaded the strait since the start of the war in late February.US forces struck a ground control station in the southern port area of Bandar Abbas, a US official said, prompting Iran to then target "the American air base that served as the source of the attack", according to IRIB, citing the country's Revolutionary Guards.The Guards did not provide details on the location of the base, though Kuwait, which hosts US troops, said its air defences were responding to an attack.Iranian foreign ministry spokesman Esmaeil Baqaei condemned the US strikes and said the Islamic republic would "take all necessary measures to defend its national sovereignty".The ministry called the strikes "violations" of the truce, though the US official described them as "purely defensive, and intended to maintain the ceasefire".The clash underscored the uncertainty surrounding the stuttering negotiations aimed at formally ending the conflict that began on February 28 with US-Israeli attacks on Iran, though neither side has appeared eager to return to all-out war.Before Thursday's strikes, Amir, a 27-year-old software developer in the Iranian capital, said fears of renewed fighting were ever present in spite of the ceasefire and talk of a deal."I feel like nothing is certain yet," he said. "The daily question is: Will there be missile strikes tonight?"- Hormuz impasse -