"Security forces raided a home in the Abi Samra neighborhood and arrested Nabil Rahim, a high-ranking member of Fatah al-Islam in Tripoli," the official said on condition of anonymity.

He added that Rahim's wife was also arrested in the dawn raid. The Abi Samra neighborhood of Tripoli is known as a hotbed for extremists.

The arrest comes days after a man purporting to be the leader of Fatah al-Islam, an Al-Qaeda-inspired militant group, threatened renewed attacks against the Lebanese army.

"Our message to the crusaders is to expect the worst. This battle was only the beginning and we will prevail," said a message posted on an Islamic website attributed to Fatah al-Islam's Palestinian chief Shaker al-Abssi.

Almost 400 people were killed, including an estimated 222 militants and 168 soldiers, in the fighting at the Palestinian refugee camp of Nahr al-Bared in northern Lebanon which ended in September after a final assault by the army.

Abssi's fate was unknown after the fighting ended although the Lebanese judiciary in October last year issued a warrant for his arrest and that of several dozen other fugitive militants.

The militia leader's wife had at one stage identified his body in a morgue, although DNA tests subsequently determined it was not him. -AFP