Islamic students slip through Indonesian police cordon outside bomb school (UPDATE)

Indonesian security forces on Wednesday raided an Islamic boarding school suspected of being a bomb factory only to find dozens of armed students they had besieged there had somehow slipped away.

The three-day stand-off ended with heavily armed police special forces running through the empty complex, the defenders having already disappeared in mysterious circumstances, according to AFP news agency.

"There was nobody there. We suspect they ran away to the mountains," national police spokesman Anton Bachrul Alam said.

The stand-off began late Monday when a suspected terrorist bomb expert accidently blew himself up during a bomb-making training session for the radical school's students. 

Dozens of students and teachers armed with swords, machetes and sticks barricaded themselves into the school grounds and refused to allow police to investigate the explosion.

Alam said more bombs were found in the complex when police finally gained access.

'When we entered the school, we found several bombs that had been disposed of,' he said.

It was unclear how the Islamist students managed to slip out of the school in Bima town, West Nusa Tenggara province, without police noticing their escape.

"We've been trying to enter the school to carry out our investigations by being persuasive, but the students are still blocking us," provincial police spokesman Sukarman Husein said earlier Wednesday.

"If persuasion doesn't work we'll have to enter the school with full force some time later today."

About 200 police and troops were deployed around the school, he said.

The Jakarta Globe reported the school was linked to radical Islamic cleric Abu Bakar Bashir, who was jailed for 15 years in June for funding a terrorist group.

The world's most populous Muslim-majority country, Indonesia is a democracy with a moderate tradition of Islam and a pluralist constitution.

But successive governments since the fall of the Suharto dictatorship in 1998 have preferred to take a soft line against radical extremists in a bid to shore up the Muslim vote at election time.

The Southeast Asian archipelago of 240 million people has witnessed several bloody terror attacks including the 2002 Bali bombings that killed more than 200 people, mostly Western tourists.