He said accepting US help would draw more fighters into the country and potentially destabilise it.
It was the first comment by the powerful anti-government leader on the military’s standoff with the Fatah al-Islam group.
|Exile in Lebanon|
As a Shia group, Hezbollah views Sunni fighters like Fatah al-Islam as enemies.
Nasrallah said the Fatah Islam fighters who attacked the military should be brought to justice.
But he said Hezbollah opposed any military incursion into the camp to crush the fighters.
He said: “The Nahr el-Bared camp and Palestinian civilians are a red line. We will not accept or provide cover or be partners in this.”
Hezbollah and its supporters accuse Fouad Siniora, the Lebanese prime minister, of being a puppet of the US and are pushing for his removal.
Nasrallah called a large airlifting of US military supplies to the Lebanese military to help in the Nahr el-Bared fight “a dangerous thing”.
He said: “Does it concern us that we start a conflict with al-Qaeda in Lebanon and consequently attract members and fighters of al-Qaeda from all over the world to Lebanon to conduct their battle with the Lebanese army and the rest of the Lebanese?”