Israeli authorities have launched a massive manhunt after six Palestinian prisoners escaped from one of the country's most secure jails overnight.

The men are believed to have dug a tunnel from their cell over several months that led to a road outside Gilboa Prison's walls.

Officials were alerted by farmers who noticed them running through fields.

The fugitives include a former leader of the militant group Al-Aqsa Martyrs' Brigade and five Islamic Jihad members.

An Israel prison service official described the escape as "a major security and intelligence failure"; Palestinian militant groups hailed it as "heroic".

The alarm was raised at Gilboa Prison, a high-security facility in northern Israel known as "the Safe", when authorities received reports from local farmers about "suspicious figures" in nearby agricultural fields.

When prison staff carried out a headcount at 04:00 (01:00 GMT), they found six inmates were missing.

The Palestinians are believed to have made their way out of the cell that they shared by digging a hole in the floor of their bathroom. The tunnel apparently emerged just outside the prison.

Photos and videos showed officials inspecting a small hole underneath a sink, and another hole in the middle of a dirt road running alongside the prison walls.

The Jerusalem Post reported that the prisoners had used a rusty spoon that they hid behind a poster.

The Shin Bet security service meanwhile said it believed they had been in contact with people outside the prison using a smuggled mobile phone and that they had been picked up in a car.

The six fugitives include Zakaria Zubeidi, a former commander of the Palestinian militant group Al-Aqsa Martyrs' Brigade in the West Bank city of Jenin, as well as five members of Islamic Jihad.

The Times of Israel said five of the six were serving life sentences in connection with deadly attacks against Israelis, while Zubeidi was in prison while on trial for two dozen crimes, including attempted murder.

Israeli border police and army troops involved in the manhunt have reportedly set up roadblocks to stop the men reaching the nearby occupied West Bank or Jordan, which is about 14km (nine miles) to the east of Gilboa prison.

Israeli Prime Minister Naftali Bennett spoke to Public Security Minister Omer Bar-Lev and "emphasised that this is a grave incident that requires an across-the-board effort by the security forces" to find the fugitives.

Islamic Jihad described the jailbreak as "heroic" and said it would "shock the Israeli defence system", while Hamas spokesman Fawzi Barhoum said it was a "great victory" that proves "the will and determination of our brave soldiers inside the prisons of the enemy cannot be defeated".