TRIPOLI, Lebanon (AFP) — A top member of an extremist Islamist group that waged a 15-week battle against the Lebanese army last year was arrested on Thursday in the northern city of Tripoli, a military official said.

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

He said Rahim's wife was also arrested in the dawn raid and that a close aide, Zakharia Trabulsi, was seized later in the day after trying to flee police.

The Abi Samra district of Tripoli is known as a hotbed of extremism.

The arrests come 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 Islamist 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 issued a warrant for his arrest and that of several dozen other fugitive militants in October last year.

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