Tehran (AFP) - 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 -

A key focus of the proposed deal has been restoring full traffic in the Strait of Hormuz, whose closure has global energy markets grappling with curbed supplies of the huge amounts of oil and gas that normally pass through it.

Oil prices bounced higher on Thursday after reports of the strikes, reversing much of the previous day’s fall spurred by hopes of a deal.

President Donald Trump threatened US ally Oman when asked about a possible short-term arrangement allowing it and Iran to control the Hormuz.

“No, the strait is going to be open to everybody,” Trump said. “It’s international waters and Oman will behave just like everybody else or we’ll have to blow them up.”

Oman had mediated talks between the US and Iran in Geneva before the war, and has itself come under attack from Tehran.

Baqaei condemned the threat towards Oman, calling it “a worrying sign of the normalisation of anarchy and intimidation in international relations”.

- Lebanon strikes -

In Lebanon, a separate ceasefire has done little to stop the fighting between Israel and Iran-backed Hezbollah, which has only escalated in recent days.

The Israeli army said on Thursday it had conducted a precise strike in the area of Beirut, with the Lebanese military saying the attack hit an apartment south of the capital.

AFPTV footage showed smoke rising from the area on the edge of Beirut’s southern suburbs, a Hezbollah stronghold.

Israel also announced new strikes on Hezbollah infrastructure around Tyre after issuing an evacuation warning to residents of the southern city.

The previous day it had declared all areas south of Lebanon’s Zahrani River – which runs roughly 40 kilometres (25 miles) from the border – as “combat zones” and told residents to leave.

Lebanese authorities said on Thursday that Israeli attacks in the south had killed at least 14 people, including three children and a soldier.

Lebanon’s health ministry on Wednesday reported the overall death toll stood at 3,269 since the start of the war, which was triggered by Hezbollah missile launches towards Israel in retaliation for the killing of Iran’s supreme leader.

The Israeli military, meanwhile, said on Thursday that a soldier was killed the day before by a Hezbollah drone near the Lebanon border. Twenty-three Israeli troops and one civilian contractor have been killed in the war so far.

Iran has insisted any agreement to end the war must apply to Lebanon.