"We insist that a ceasefire in Lebanon is an essential condition for any deal aimed at ending the war," foreign ministry spokesman Esmaeil Baqaei said at a weekly press briefing, as Israel expands its offensive in Lebanon.

Israeli Prime Minister Benjamin Netanyahu has vowed to push deeper into Lebanon, while the UN Security Council is set to hold an emergency meeting on Monday after Israel's military took control of the medieval Beaufort castle.

Baqaei said Iran "will take all measures to support Lebanon and the resistance against the Zionist regime's illegal aggression".

His remarks come as Tehran and Washington continue exchanging messages in an effort to finalise a framework agreement aimed at ending the war, which erupted with US-Israeli strikes on Iran in late February and subsequently engulfed the region.

A fragile ceasefire between Iran, the United States and Israel has been in place since April 8, but military confrontations have continued in Lebanon despite Tehran's insistence that Lebanon should be covered by the truce.

On Monday, Baqaei accused the United States of "violating the ceasefire" following a brief overnight flare-up in which the US struck a telecommunications tower in a southern Iranian port city.

Iran's Revolutionary Guards said they targeted the US base from which the attack originated, without specifying its location.

Iran will "take whatever measures we deem necessary to defend Iran's national security", Baqaei said.

He said Iran was not seeking concessions in its exchanges with Washington but rather the fulfilment of its rights, including the release of assets frozen abroad under US sanctions.

He also said details related to Iran's nuclear programme -- a key sticking point for Washington -- have not yet been part of the exchanges.

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

He added that the exchange of messages with the United States was continuing, but that "we have not yet reached a final conclusion".

"We will decide on the arrangements for signing at the appropriate time," he added.