In a statement on Saturday, Mr Gallant's office said he was conducting "an operational situation assessment" regarding what it called "the expansion of IDF (Israel Defence Forces) activities in the northern arena".

Israeli tanks and troops were later pictured near the border, in what Sky News' security and defence editor Deborah Haynes said is the "clearest sign yet" that Israel's conflict with Hezbollah is "about to expand even further".

Israel-Hezbollah latest: Hassan Nasrallah killed

The military said it was mobilising three more battalions of reserve soldiers to serve across the country. It had already sent two brigades to northern Israel to prepare for a possible ground invasion.

The deployment comes after Hezbollah confirmed that its leader of more than three decades Hassan Nasrallah was killed in an Israeli airstrike in Beirut on Friday.

The militant group - which is aligned with Iran - vowed to continue its fight against Israel even as attacks continued to bombard areas around Lebanon's capital.

At least six people were killed in the strikes - not including Nasrallah - and 91 were wounded, according to preliminary figures from Lebanon's health ministry.

The United Nations high commissioner for refugees said that airstrikes led to the displacement of "well over 200,000" people inside Lebanon.

"More than 50,000 Lebanese people, and Syrians living in Lebanon, have crossed the border into Syria," Filippo Grande wrote on X on Saturday.

Approximately 60,000 Israelis have been evacuated from their homes along the Lebanese border for almost a year.

'Israel is on the move'

In his first public remarks since the killing, Israeli Prime Minister Benjamin Netanyahu described Nasrallah as a "terrorist" and said his killing would help bring displaced Israelis back to their homes in the north and would pressure Hamas to free Israeli hostages held in Gaza.

But with the threat of retaliation high, he said the coming days would bring "significant challenges" and warned Iran against trying to strike.

"There is no place in Iran or the Middle East that the long arm of Israel will not reach, and today you already know how true this is," Mr Netanyahu said.

"We have great achievements, but the work is not yet complete. In the coming days we will face significant challenges, and we will face them together," he added.

"We are determined to continue to strike at our enemies, return our residents to their homes, and return all our abductees. We do not forget them for a moment.

"Israel is on the move."

Iranian Supreme Leader Ayatollah Ali Khamenei condemned the killing and announced five days of mourning. He said Lebanon will make Israel "regret their actions" and Nasrallah's blood "will not go unavenged".

In a letter to the UN Security Council, Iran's UN ambassador Amir Saeid Iravani asked for an "emergency meeting" of the 15-member body, calling on it to "compel Israel" to cease all military action in both Gaza and Lebanon and "comply with relevant UNSC resolutions".

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Meanwhile, hundreds of protesters took to the streets of Tehran, waving Hezbollah flags and chanting "death to Israel" and "death to Netanyahu the murderer".

People also gathered in the Lebanese city of Sidon and in Amman, Jordan, to mourn Nasrallah. The 64-year-old had countless followers across the Arab and Islamic world, but was viewed as an extremist in much of the West.