The UN has warned the deaths of four of its personnel in southern Lebanon may deter countries from contributing to a future peacekeeping force in the area.
The UN said it wanted to "get to the bottom" of what happened
UN deputy chief Mark Malloch-Brown said they accepted Israel's apology for the losses to Israeli fire, but still had "serious concerns" about what happened.
The UN has called for a three-day truce to let aid enter Lebanon, but Israel has rejected the request.
US Secretary of State Condoleezza Rice returns to the region on Saturday.
She is expected to lobby for a UN Security Council resolution that would lead to an international force being deployed in southern Lebanon.
The UN says children, elderly and disabled have been left stranded and supplies are "running out very, very fast" in southern Lebanon after two weeks of fighting.
But an Israeli government spokesman said there was no need for a temporary ceasefire because Israel had opened a humanitarian corridor to and from Lebanon.
Avi Pazner accused Hezbollah of deliberately preventing aid from reaching the area.
Correspondents say there is concern in Israel that Hezbollah might use such a truce to replenish its stock of weapons.
In other developments:
- Lebanon has said an Israeli attack on fuel tanks at a power plant has created the biggest environmental disaster the Mediterranean region has known
- A bridge has been destroyed in the eastern Bekaa valley as the Israeli air force continues its bombardment of Lebanon
- Several rockets have been launched at the northern Israel town of Safid amid further fire by Hezbollah.
The UN says some 600 people have been killed by Israeli action in Lebanon, of which about a third were children.
Fifty-one Israelis, including at least 18 civilians, have been killed, mostly by Hezbollah rockets.
The Israeli assault began after Hezbollah captured two Israeli soldiers and killed eight in a cross-border raid on 12 July.
Israel said Tuesday's deaths in a strike on a UN base were an accident - but UN officials said they had contacted Israel a dozen times before the bombing and asked them to stop firing, which Israel did not.
Mr Malloch-Brown told the BBC the UN "continued to harbour serious concerns about what went on in the Israeli military forces that day".
He said the losses posed a "very serious threat to the whole concept of neutral peacekeeping.
"Peacekeeping is a dangerous business and we depend on the support of the international community," he said.
"When people die it is not a simple accident to be brushed away."
The UN wants the injured to be evacuated during a truce
The UN Security Council issued a statement voicing "shock and distress" at the deaths, after the US blocked calls for harsher criticism of Israel.
Mr Malloch-Brown said Washington would have to "think hard" about the consequences of its move for the recruitment of an international force.
UK Prime Minister Tony Blair said world leaders would discuss the deployment of a "stabilisation force" at a meeting at the UN headquarters in New York on Monday.
UN Secretary-General Kofi Annan said countries who may be in a position to contribute troops would attend the meeting on the proposal, which is due for discussion by the Security Council later next week.