Israel’s military chief has said the country is preparing for a possible ground assault in Lebanon as the army said it was calling up two brigades of reservists and a deadly aerial bombardment of Lebanon continued for a third day.

The latest wave of air strikes was aimed at “preparing the ground for your possible entry”, Herzi Halevi told soldiers in northern Israel on Wednesday, in the most explicit reference to a potential ground operation against the Lebanese armed group Hezbollah since the Israeli air force on Monday dramatically stepped up air attacks on Lebanon.

“We are preparing the process of a manoeuvre, which means your military boots, your manoeuvring boots, will enter enemy territory, enter villages that Hezbollah has prepared as large military outposts,” Halevi said.

Hezbollah, an Iran-aligned group, “expanded its range of fire, and later today, they will receive a very strong response. Prepare yourselves,” Halevi added, referring to a missile launch that Hezbollah says targeted Mossad’s headquarters near Tel Aviv.

Earlier, the Israeli army said it was calling up two brigades of reservists to the north to keep fighting Hezbollah and allow the return to the north of thousands of civilians displaced by crossfire – the stated aim of this week’s deadly offensive into Lebanon.

In a brief video message on Wednesday, Israeli Prime Minister Benjamin Netanyahu repeated his promise to return the displaced Israelis and said Hezbollah is being hit “harder than it could imagine”.

“I can’t go into detail about everything we do. But I can tell you one thing: We are determined to return our residents in the north safely to their homes,” he said.

Lebanon’s health minister said at least 51 people were killed on Wednesday in the continuing Israeli strikes, raising the death toll from the past three days to 615, with more than 2,000 wounded.

Earlier in the morning, Hezbollah fired dozens of missiles at Israel, including a longer-range projectile that set off air raid sirens in Tel Aviv and across the centre of the country. Israel said that it was the first time a missile had reached the central area and that the projectile was intercepted. No casualties or damage were reported, it added.

Hezbollah said it had fired a ballistic missile at the headquarters of Israel’s Mossad intelligence agency, which it said was responsible for the targeted killing of its senior leaders. Israel later said it struck the site from which the missile was launched in southern Lebanon.

Tens of thousands flee

More than 90,000 people have fled the south of the country to seek shelter in the north, according to the International Organization for Migration (IOM).

Long queues of cars fleeing southern towns have jammed the streets, aid groups have called for blood donations and schools have been turned into shelters.

The IOM said the World Health Organization and the Lebanese Ministry of Public Health are preparing for mass casualty events in Lebanon and maintaining essential services. These include mental health support despite health stock levels being critically low, and urgent restocking being needed, it added.

The fire exchanges have alarmed world leaders, triggering a diplomatic frenzy. US President Joe Biden said an all-out war was possible in the region, but there was also still room for diplomacy. “An all-out war is possible, but I think there’s also the opportunity – we’re still in play to have a settlement that can fundamentally change the whole region,” Biden said in an appearance on the  ABC programme The View, broadcast on Wednesday.

The United States is leading a new diplomatic effort to end hostilities in both Gaza and Lebanon, linking the two conflicts as part of a single initiative, the Reuters news agency reported, citing seven sources familiar with the initiative. Details are being hammered out at the UN General Assembly in New York, read the report. It would be the first time the two fronts are linked as part of a US diplomatic push, the sources said.

Since Israel launched its assault on Gaza on October 7, Hezbollah and Israel have been trading near-daily rocket fire, causing the displacement of tens of thousands of people living near the border. But while those exchanges were largely confined to frontier areas and were aimed mostly at military targets, tensions escalated dramatically last week after Israel said it would pivot its military focus to its northern frontier.

A mass detonation last week of pagers and other communication devices owned by Hezbollah members killed at least 42 people and injured more than 3,000. Israel did not deny or confirm responsibility for these attacks. This week, it dealt a blow to the group’s military leadership by killing in two separate attacks the head of its missiles division, Ibrahim Qobeissi, and the second-in-command of its elite Radwan unit, Ibrahim Aqil.