As many as six Palestinian militants on Monday broke out of a high-security Israeli prison in what Prime Minister Naftali Bennett called a grave incident.

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

It is reportedly said that 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.

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

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

Palestinian prisoners organisation said that four of the men were serving life sentences. 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 militant group Hamas, Reuters reported.

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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.

Meanwhile, a police spokesman said that 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.

Zakaria Zubeidi, one of the escaped militants, is 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.