Weeks of indirect talks marked by stark threats and several waves of air strikes have so far failed to agree an end to the war or the reopening of the Strait of Hormuz, the key shipping channel for Gulf oil and gas.

The latest exchange of fire coincided with Israel expanding its offensive into Lebanon, with Prime Minister Netanyahu vowing to push deeper into the country and instructing the military to strike "terror targets" in a southern district of Beirut.

The United States has backed its ally's operations in Lebanon against the Iran-backed group Hezbollah, while still trying to come to an agreement with Iran to end the war it launched in late February with strikes on Tehran, reopen Hormuz and impose controls on Iran's nuclear programme.

But Iran again said Monday it had not yet reopened any nuclear negotiations and insisted that Israel must halt its offensive in Lebanon before any wider deal to end the war can be agreed.

The US naval blockade on Iran's ports and the escalation in Lebanon were "clear evidence of US non-compliance with the ceasefire", Iran's chief negotiator Mohammad Bagher Ghalibaf posted on X.

- 'Essential conditions' -

"We know when it is necessary to act on nuclear matters," foreign ministry spokesman Esmaeil Baqaei told a weekly news briefing.

"No negotiations have taken place on the details of the nuclear file. At this stage, our priority is ending the war."

"We insist that a ceasefire in Lebanon is an essential condition for any deal aimed at ending the war," Baqaei said, adding: "The United States is also violating the ceasefire, including this morning."

The US military said it had carried out "self-defence strikes" on Iranian radar and drone control sites over the weekend -- its third such wave in just over a week -- this time in response to the downing of a US MQ-1 drone.

Shortly afterwards, Iran's Revolutionary Guards told state media they had targeted an airbase used by the US military from which the attack originated.

The Guards did not identify the country said to be hosting the base, but the Kuwaiti military said its air defence had intercepted "hostile missile and drone attacks".

- Sticking points -

Iran was already in talks with the United States about the fate of its nuclear programme in February, when the US and Israel launched air and missile strikes that wiped out much of the Islamic republic's senior leadership and plunged the Middle East into war.

While Tehran has long insisted that its nuclear programme is for purely civilian ends, the United States and its Western allies suspect it aims to develop am atomic weapon.

Late Sunday, Trump posted on social media that the deal under discussion "states, very clearly, that Iran will not have a Nuclear Weapon".

Iran has said it needs the release of $12 billion in frozen assets before engaging in substantive talks on its nuclear programme, and dismissed earlier Trump comments suggesting that its stockpile of enriched uranium would be destroyed.

- Lebanon front -

A truce in Lebanon between Israel and Hezbollah formally began on April 17 but it has never been observed, with both sides accusing each other of violating it.

Israel has pursued a ground offensive into southern Lebanon, raising its flag over a medieval castle that served as a base during its two-decade occupation of the country in the 1980s and 1990s.

Netanyahu called the retaking of the Beaufort stronghold "a dramatic shift" and vowed to continue the fight against Hezbollah, with strikes and incursions ever deeper into Lebanon.

The UN Security Council will hold an emergency meeting Monday on the widening Israeli offensive, diplomatic sources told AFP.

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