Singapore, which prides itself on rigorous anti-terrorist measures, today blamed a security lapse for the escape of an alleged leader of the Jemaah Islamiah militant network.

Authorities mounted a massive manhunt as analysts said Mas Selamat Kastari, accused of planning to hijack a plane and crash it into the city's Changi Airport, would try to flee to Indonesia.

"This should never have happened. I'm sorry that it has," Home Affairs Minister Wong Kan Seng told parliament.

Rohan Gunaratna, head of the International Centre for Political Violence and Terrorism Research, said Mas Selamat had escaped once when he was in Indonesia, but his flight from the Singapore facility was unexpected.

"I think that it has surprised the Singapore authorities and everyone because Singapore's security systems are world-class," he said.

He walks with a limp

The ministry said Mas Selamat walks with a limp and was not known to be armed.

It said he escaped on Wednesday afternoon from the Whitley Road Detention Centre, which holds prisoners detained by the Internal Security Department.

More than 24 hours later he remained at large, the ministry said.

Wong said Mas Selamat disappeared after he was taken from his cell to the family visit room to await his relatives.

"He asked to go to the toilet, where he escaped."

The minister was responding to a member of parliament who said the escape "raised concerns about our police force vigilance and security operating procedures."

Wong said security had been tightened at all air, sea and land checkpoints and no effort would be spared to locate Mas Selamat, who analysts say received military training in Afghanistan.

Manhunt launched

Soldiers, Special Operations police and paramilitary Nepalese Gurkhas had joined the manhunt. Police in neighbouring Malaysia said they had also stepped up border security.

Businesses, offices and schools operated normally, despite the massive security presence near the detention centre.

Clive Williams, a professor with the Australian Defence Force Academy, predicted Mas Selamat would try to reach Indonesia, "because if he stays in Singapore he's going to be arrested fairly quickly I would think."

Gunaratna also said Mas Selamat, a Singaporean citizen, would try to reach Indonesia, whose nearest islands are clearly visible from the city-state.

Mas Selamat, born in 1961, was arrested on the Indonesian island of Bintan near Singapore in 2003 and sentenced by a court there to 18 months in jail.

He was later released but arrested again by Indonesian authorities in January 2006 before being handed over to Singapore.

Mas Selamat had fled Singapore in December 2001 following an Internal Security Department operation against JI, Singapore's home affairs ministry has said.

In that operation, Singapore authorities arrested 15 people, 13 of whom were suspected JI members, who allegedly planned to attack a bus carrying Americans to a subway station.

'No imminent danger'

The Home Affairs Ministry has said Mas Selamat was among those who planned retaliation against Singapore for the arrests and detention of his fellow JI members.

Gunaratna called Mas Selamat a ruthless and "very determined terrorist" closely associated with Noordin Muhammad Top, who has spent years on the loose and was believed by analysts to have broken from JI to form a more hardline faction.

Sidney Jones, an analyst who has extensively studied JI, said Mas Selamat has little power left within the group, whose Indonesian leadership now rejects "Al-Qaeda tactics" targeting foreigners.

The home affairs minister said Singapore's security agencies concluded there was "no imminent danger" to the public after Mas Selamat's escape.

Singapore, a staunch US ally, has said it is a top target for extremists and has taken elaborate security measures to prevent an attack. Warnings for people to be vigilant are broadcast on the city's subway system.