Israeli Prime Minister Benjamin Netanyahu said on Monday that the military was changing the "security balance" along the northern border after launching the deadliest strikes on south Lebanon since fighting started last October.

"I promised we would change the security balance, the balance of power in the north –- and that is exactly what we are doing," Netanyahu said during a security meeting, adding that Israel's policy was to preempt threats rather than "wait" for them.

French presidential envoy for Lebanon Jean-Yves Le Drian will arrive in Beirut on Monday, media reports said.

The United Nations voiced alarm Monday at the escalating violence between Israel and Hezbollah in Lebanon, warning that actions and rhetoric was catapulting the Mideast conflict "to another level".

"We are extremely concerned, deeply worried about the escalation in Lebanon," Ravina Shamdasani, spokeswoman for the UN rights office, told AFP.

Iran's foreign ministry warned Israel on Monday of "dangerous consequences" following deadly strikes on Hezbollah strongholds in Lebanon.

Foreign ministry spokesman Nasser Kanani called the Israeli strikes "insane", and warned of "the dangerous consequences of the Zionists' new adventure".

Thousands of people are leaving south Lebanon and heading north after Israel intensified airstrikes on Monday, leaving 100 people dead and hundreds wounded.

In the southern port city of Sidon, the main north-south highway was packed with cars heading north in the direction of the capital Beirut.

State-run Lebanese media said Israel was conducting new raids across Lebanon's east Monday, hours after deadly strikes on the south and east, in the deadliest single day in nearly a year of cross-border clashes.

"Enemy raids targeted the mountain" areas in the Baalbek region and several towns and villages, the National News Agency said, adding that "the Israeli enemy" launched a strike on the town of Hermel.

The United Nations Special Coordinator for Lebanon, Jeanine Hennis-Plasschaert, begins an official visit to Israel today, Monday, her office said.

She is set to meet with senior Israeli officials to discuss recent developments, the office said in a statement.

Grand Ayatollah Ali Sistani, Shiite Islam's highest authority in Iraq, appealed Monday for "every possible effort" to end Israeli "aggression" against Lebanon.

Sistani called for "the exercise of every possible effort" to end this "barbaric aggression and to protect the Lebanese people."

A missile hit the al-Wardiyet area between the Jbeil district towns of Almat and Ehmej on Monday, as Israeli warplanes targeted Lebanon’s western mountain range.

Hezbollah said it targeted three sites in northern Israel including military production facilities in retaliation for intense Israeli strikes on south and east Lebanon.

"In response to the Israeli enemy attacks that targeted the south and Bekaa areas", Hezbollah fighters bombed two north Israel military positions as well as the "Rafael defense industry complexes" north of the city of Haifa, the group said in a statement.