The leader of the Shia militant group Hezbollah has urged Lebanon's government not to storm a refugee camp to root out Sunni radicals.
Sheikh Nasrallah said Hezbollah opposed Fatah al-Islam
Sheikh Hassan Nasrallah said Lebanon should not become part of the American war against al-Qaeda.
The Sunni militants, alleged to have links to al-Qaeda, have taken over the Nahr al-Bared camp near Tripoli.
But in a TV address Hezbollah's leader said the conflict could be solved politically and should not escalate.
And he outlined Hezbollah's opposition to any militant incursion into the camp.
"The Nahr el-Bared camp and Palestinian civilians are a red line," Sheikh Nasrallah said.
"We will not accept or provide cover or be partners in this."
The Lebanese should not allow themselves to become entangled with al-Qaeda on behalf of the US, he added.
Earlier, several transport planes carrying military aid for the Lebanese army from the US and its Arab allies arrived at Beirut airport, following a request by the Lebanese government.
The military supplies are believed to include ammunition for automatic rifles and heavy weapons, spare parts for military helicopters and night-vision equipment.
US Secretary of State Condoleezza Rice has reiterated Washington's support for the Lebanese government.
She said gunmen in Nahr al-Bared were trying to destabilise a democratic government.
But Sheikh Nasrallah said he doubted the sincerity of the sudden US interest in Lebanese internal affairs.
"I wonder why all this care now for the Lebanese army," he said, referring to the 2006 conflict between Israel and Hezbollah.
Thousands of people have fled the refugee camp as aid workers struggle to deliver food and medicine to thousands still inside.
At least 50 soldiers and militants have been killed in the fighting at the camp so far. The civilian death toll is not known.
The fighting at the camp is the bloodiest internal conflict in Lebanon since the civil war ended 17 years ago.
It began after security forces raided a building in Tripoli to arrest suspects in a bank robbery. Fatah al-Islam militants then attacked army posts at the entrances to the camp.
A large force of Lebanese troops hit back, bombarding the camp and storming a building on the outskirts of Tripoli.
Fatah al-Islam is a radical Palestinian splinter group alleged to have links with al-Qaeda. Lebanese officials also believe it is backed by Syria.
Other Palestinian factions have distanced themselves from the group, which emerged last year.