Israel's military says it is striking Hezbollah targets in southern Lebanon and that the chief of staff has approved plans for the area near the border of the two countries.

At the same time, warning sirens have rung out in northern Israel, indicating possible incoming fire.

"The Hezbollah terrorist organisation has turned southern Lebanon into a combat zone," the Israel Defense Forces (IDF) said.

"For decades, Hezbollah has weaponised civilian homes, dug tunnels beneath them, and used civilians as human shields. 

"The IDF is operating to bring security to northern Israel in order to enable the return of residents to their homes, as well as to achieve all of the war goals."

The strikes began just before a televised address by Hezbollah leader Hassan Nasrallah.

His speech followed two day of sweeping attacks on Hezbollah's communications systems, which he said "crossed all red lines".

He said the attacks, during which pagers and walkie-talkies used by members of the miltant group, would be met with "just punishment". But he did not elaborate.

Lebanon's health ministry said the exploding devices killed 37 people and wounded about 3,000 others.

Hezbollah has blamed Israel for the attacks, but it has neither confirmed nor denied being behind them.

"There is no doubt that we have been subjected to a major security and military blow that is unprecedented in the history of the resistance and unprecedented in the history of Lebanon," Nasrallah said.

"The enemy went beyond all controls, laws and morals," he said, adding the attacks "could be considered war crimes or a declaration or war".

"They could be called anything and they deserve to be called anything. Of course, that was the intention of the enemy."

Israeli jets created sonic booms over Beirut during the speech.

Nasrallah said Hezbollah would not stop fighting Israel on Lebanon's southern border until the war in Gaza ended.

Iranian state media reported that the head of Iran's Revolutionary Guards, Commander Hossein Salami, told Nasrallah that Israel would face "a crushing response from the axis of resistance".

The IDF confirmed that two soldiers were killed in Israel's north on Thursday, local time.

Israeli news outlets reported one was killed by a drone and the other by an anti-tank missile fired by Hezbollah across the border.