JERUSALEM: Six Palestinians broke out of a high-security Israeli prison on Monday in what Prime Minister Naftali Bennett called a grave incident.

Israeli police and the military had started a search after the escape from Gilboa prison in northern Israel.

Five of the fugitives belong to the Islamic Jihad movement and one is a former commander of an armed group affiliated with the mainstream Fatah party, the Prisons Service said.

Arik Yaacov, the service's northern commander, said the escapees appeared to have opened an hole from their cell toilet floor to access passages formed by the prison's construction.

The facility, about 4 km (2 miles) from the boundary with the occupied West Bank, is one of the highest-security jails in Israel and houses Palestinians convicted or suspected of anti-Israeli activities, including deadly attacks.

Four of the men were serving life sentences, a Palestinian prisoners organisation said.

Several Palestinian factions hailed the jailbreak.

"This great victory proves again that the will and determination of our brave soldiers inside the prisons of the enemy cannot be defeated," said Fawzi Barhoum, a spokesman for the Islamist group Hamas.

Bennett's office said he spoke with Israel's internal security minister and "emphasised that this is a grave incident that requires an across-the-board effort by the security forces" to find the escapees.

A police spokesman said security forces believed the fugitives might try to reach the West Bank, where the Palestinian Authority exercises limited self-rule, or the Jordanian border some 14 km (9 miles) to the east.

One of the escapees was identified by the Prisons Service as Zakaria Zubeidi, a former commander of Fatah's Al Aqsa Martyrs Brigades in the West Bank city of Jenin. The brigades carried out deadly attacks against Israelis during a 2000-2005 Palestinian uprising.