Militants from a radical Islamist group which fought deadly battles with the Lebanese army in 2007 escaped from the country's main jail on Saturday, a security official told AFP.
"Five inmates have escaped from Ward D in the Roumieh prison," northeast of Beirut, the official said on condition of anonymity. "We believe at least three of them belong to Fatah al-Islam."
He said the hunt for the inmates was ongoing, while television footage showed troops had surrounded the notorious prison as a helicopter flew overhead.
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Lebanon's state-run news agency reported that one of the escaped inmates was a Sudanese national.
Roumieh, the oldest and largest of Lebanon's overcrowded prisons, has witnessed sporadic prison breaks in recent years and escalating riots over the past months as inmates living in dire conditions demand fairer treatment.
In the summer of 2007, Al-Qaeda inspired Fatah Al-Islam group led an uprising against the army in the Palestinian refugee camp of Nahr al-Bared in northern Lebanon.
The fighting killed some 400 people, including 168 soldiers, and deadly clashes also broke out in the nearby port city of Tripoli but some Islamist leaders escaped despite a 15-week army siege of the camp.
The militant group is also accused of being behind twin bus bombings in a Christian suburb northeast of Beirut that left three dead and close to 20 wounded in 2007.