GENEVA (AFP) - UN humanitarian chief Jan Egeland has warned that fuel supplies in Lebanon could run out within days or weeks, paralyzing hospitals and shutting down electricity across the country.

"The fuel situation is the single most worrying humanitarian crisis at the moment," Egeland told a news conference Thursday.

"If there's one thing that will be the most critical -- even more critical than food -- over the next days and weeks, it's fuel," he said.

Fuel supplies have been stifled by a combination of Israeli raids that have destroyed roads and bridges -- halting convoys of urgently needed relief supplies too -- and the inability to import more fuel into Lebanon.

Four hospitals in the south of the country have already run out of fuel needed for generators that were crucial for surgery and stocking drugs, Egeland said.

But a broader problem was also developing after the owners of two oil tankers chartered by the UN stopped them docking in Beirut because conditions were regarded as too dangerous, he added.

"The national electricity grid will even halt if there is no fuel coming in soon, and the two UN-supported tankers will help in this," Egeland told a news conference.

Israeli forces have bombed major power stations, many of which are oil-fired.

The World Health Organization said it had purchased enough fuel to keep hospitals in the south going for 10 days "and is ready to deliver it".

"However, the security situation continues to make delivery extremely difficult," it added in a statement.

The UN humanitarian coordinator slammed Israel and the Lebanese Hezbollah for their conduct in the conflict in Lebanon and northern Israel, condemning a "senseless cycle of violence". "It's a disgrace really because the parties to the conflict, Israel and Hezbollah, could give access in a heart beat, and then we could help 120,000 people in southern Lebanon."

Ten requests for relief or reconnaissance convoys were made Thursday, and Israeli forces granted security clearance for eight of them, the humanitarian chief said.

But no requests were even attempted for the virtually isolated port of Tyre after days of refusals, Egeland said.

He also called into doubt the value of the notification system with Israeli forces following air strikes that came perilously close to relief convoys.

"This gives us the impression it is a very fragile system... It is a system which is not working," he said.

About one quarter to one third of Lebanon's population was fleeing, according to the UN.

Egeland cautioned that under any subsequent end to the fighting, the Lebanese must be able to return to their homes.

"The people have a right to return. If the right is not granted, they will be sources of perpetual bitterness, perpetual hatred, perpetual conflict," he added.