Six Palestinian prisoners escaped from Israel's northern Gilboa Prison early Monday, Israeli media reported.
State broadcaster Kan quoted the Israeli Prisons Service as saying that the detainees tunneled out of the high-security prison.
It said five of the prisoners belonged to the Islamic Jihad movement, and one was a former commander of the Fatah group.
Israel's daily Maariv newspaper reported that a police contingent had arrived at the prison, and a security operation was launched.
All the inmates were serving life sentences, according to the Israeli website Walla news.
Haaretz newspaper said the six were cellmates and the tunnel they dug reached dozens of meters in depth.
In a statement, Israeli police said it has launched a manhunt for the six detainees.
According to the statement, police erected roadblocks in the prison surroundings and launched an operation to hunt down the escapees.
Both Hamas and Islamic Jihad groups hailed the prison break.
In a statement, Hamas spokesman Fawzi Barhoum termed the prison break as a “big victory”.
"This victory proves again that the will and determination of our prisoners in the [Israeli] prisons are invincible and undefeatable whatsoever the challenges are," said Barhoum.
Islamic Jihad said the escape of the detainees was a "resounding defeat" to Israel.
"This heroic operation will deepen the [Israeli] occupation failure and its inability," it added in a statement.