UNITED NATIONS, May 22 (Reuters) - U.N. Secretary-General Ban Ki-moon denounced on Tuesday "criminal attacks" against Lebanese troops fighting Islamist militants in a Palestinian refugee camp and urged immediate access for aid to civilians.

"The Secretary-General deplores the criminal attacks carried out over the past several days against the Lebanese Army and security forces," a statement said.

"These actions constitute an assault on Lebanon's stability and sovereignty, and have seriously endangered civilians. They must halt immediately."

Thousands of Palestinians fled the camp in north Lebanon on Tuesday when a fragile truce eased the fighting. Aid groups arrived to deliver supplies, but some U.N. aid trucks withdrew when shells exploded near their convoy.

Ban and U.N. aid chief John Holmes deplored that attack, which Holmes termed "outrageous and completely unacceptable", and called for safe corridors to be set up for supplies and medical staff.

"We are clearly in the face now of a serious humanitarian problem for the people trapped inside the camp," Holmes told reporters.

"I would appeal to all those engaged in this to respect the fact that this is a civilian camp and that they have an obligation to respect international humanitarian law by protecting civilian lives and not using heavy weapons indiscriminately."

The firm tone of Ban's statement reflected support in the Security Council for the Lebanese government. The council appears set to establish a special court to try suspects in the murder of former Lebanese prime minister Rafik al-Hariri, despite objections by Lebanese opposition politicians.

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