The United States and Iran accused each other of violating an ongoing truce on Thursday following an exchange of fire, three months after the Middle East war began with a wave of US-Israeli strikes on the Islamic republic.The latest exchange was the most serious since the ceasefire started in April, rattling ongoing efforts to negotiate an end to the war and drawing in US ally Kuwait, which accused Iran of a "dangerous escalation".It came as violence on the war's Lebanese front escalated sharply after Israel declared much of the country's south a combat zone and stepped up strikes, including near Beirut.Iranian forces had fired at four ships attempting to cross the Strait of Hormuz without authorisation, state broadcaster IRIB reported on Thursday. Iran has blockaded the waterway since the start of the war on February 28.US forces said they had intercepted five attack drones in and around the strait, and prevented the launch of a sixth from a ground control station in the southern port area of Bandar Abbas.The strike in Bandar Abbas prompted Iran to target "the American airbase that served as the source of the attack", according to IRIB, citing the country's Revolutionary Guards.The Guards did not specify the location of the base, but Kuwait, which hosts US troops, said its air defences had responded to incoming fire.Its foreign ministry later condemned "the criminal Iranian attacks that targeted the territory of the State of Kuwait with missiles and drones, in a dangerous escalation".The US Central Command called the attack an "egregious ceasefire violation".Iranian foreign ministry spokesman Esmaeil Baqaei, for his part, said the Islamic republic would "take all necessary measures to defend its national sovereignty", likewise describing the US strikes as "violations" of the truce.A US official told AFP the actions had been "measured" and "intended to preserve the ceasefire".Iran's Guards threatened a "firm response" on Thursday in the event of renewed attacks.The clash underscored the uncertainty surrounding the stuttering negotiations aimed at formally ending the conflict, 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 -