The UN's Jan Egeland has condemned the devastation caused by Israeli air strikes in Beirut, saying it is a violation of humanitarian law.
Mr Egeland, the UN's emergency relief chief, described the destruction as "horrific" as he toured the city.
He arrived hours after another Israeli strike on Beirut. Israel also hit Sidon, a port city in the south crammed with refugees, for the first time.
In Haifa, two people died as Hezbollah rockets struck the Israeli city.
Fifteen people were reportedly injured by the volley of rockets, which struck a house and an industrial zone.
The BBC News website's Raffi Berg visited the scene of one of the rocket attacks in northern Haifa.
Several were injured in rocket attacks on Haifa
He says the rocket exploded next to a carriageway, raking passing cars with shrapnel and ball bearings and killing a man in a nearby vehicle.
A later barrage of missiles was reported to have injured five people.
'Block after block'
Mr Egeland arrived in southern Beirut on Sunday just hours after Israeli strikes on the Hezbollah stronghold.
A visibly moved Mr Egeland expressed shock that "block after block" of buildings had been levelled.
He said the "disproportionate response" by Israel was a "violation of international humanitarian law".
He appealed for both sides to halt attacks and said UN supplies of humanitarian aid would begin to arrive in the next few days.
"But we need safe access," he said. "So far Israel is not giving us access."
Israel has said it will lift its blockade on Beirut's port to allow aid through, but with roads, bridges and trucks among Israel's targets, transporting it around the country is difficult.
In other developments:
- UK Foreign Minister Kim Howells is due to meet Israeli Foreign Minister Tzipi Livni. A day after accusing Israel of targeting "the entire Lebanese nation", he said the British government understood Israel's need to defend itself and criticised Hezbollah for hiding weapons in civilian areas.
- The US Secretary of State Condoleezza Rice is due to leave for the Middle East later on Sunday.
- Israeli Defence Minister Amir Peretz said Israel supports the idea of an international peacekeeping force in south Lebanon, and suggested it should be led by Nato. A Nato official said there had been no discussion so far of any Nato role.
- Iranian president Mahmoud Ahmadinejad said Israel had "pushed the button for its own destruction".
- Syria's information minister said his country would enter the conflict if a major Israeli ground invasion of Lebanon threatened the security of Damascus.
- An unarmed UN observer was seriously wounded during fighting between Israeli forces and Hezbollah fighters in the village of Maroun al-Ras, which Israel said it had taken control of on Saturday.
- The French and German foreign ministers are also in Israel for talks on the crisis.
Israel's bombing campaign continued, with strikes on Beirut and on southern and eastern Lebanon in the early hours of Sunday.
The Associated Press news agency reported at least eight deaths on Sunday - an eight-year-old boy, a Lebanese photographer, three civilians fleeing in a minibus, and three Hezbollah fighters.
One target was the southern port of Sidon, a city not previously targeted by Israel, where 42,000 refugees from the surrounding area have flooded in the hope of safety.
The BBC's Roger Hearing in the city reports that a mosque was destroyed in one strike, which hit less than 500m (550 yards) from a hospital. At least four people were injured.
While Israel said the mosque was a meeting place for Hezbollah militants, local doctors insisted it was just "a place for prayers".
The BBC's Jim Muir in the southern city of Tyre reports intense bombardment, with Hezbollah firing missiles from the area and Israel launching air strikes in retaliation.
At least 15 civilian vehicles have been hit on the roads, including one taking injured people to a nearby hospital, he says.
Israeli air strikes hit civilian cars near Tyre
Further east, more Israeli air strikes forced engineers to turn back who were trying to repair impassable roads so a UN-escorted aid convoy could get through, our correspondent reports.
He says that bombing has intensified in the region since Israel dropped warning leaflets on Friday, and the Israelis are now shooting at almost anything on moving on the roads.
At least 364 Lebanese have been killed in the 12 days of violence, many of them civilians, and angry protests condemning Israeli attacks have been held in cities around the world.
At least 36 Israelis have been killed, including 17 civilians killed by rockets fired by Hezbollah into Israel.