A "security lapse" led to the escape of the leader of the city-state's wing of the Islamic militant network, Jemaah Islamiah (JI), Singapore said on Thursday.

Wong Kan Seng, Singapore's minister for home affairs, told members of parliament that Mas Selamat bin Kastari escaped from the toilet of a police detention facility, in the middle of a posh residential area, on Wednesday afternoon.

Acknowledging that the security lapse "should have never happened", Wong said everything was being done to arrest Kastari.

Singapore security systems are generally tight and sophisticated on the island to police its borders.

Local newspapers reported that thousands of policemen were deployed late on Wednesday in a massive hunt for Kastari, who was thought to have escaped unarmed and walks with a limp.

The JI has been blamed for several deadly bombing attacks in Southeast Asia, including the 2002 bombings that killed more than 200 people on Indonesia's resort island of Bali.

Singapore, a strong U.S. ally and a major base for Western businesses, sees itself as a prime terrorist target in the region after it foiled JI plots in 2001 to attack its airport and other targets, including the U.S. embassy, the Americana Club and the Singapore American School.

Kastari left Singapore after the plots seven years ago after nearly 40 other suspected members of JI were rounded up. He was arrested by the Indonesian police on the Indonesian island of Bintan in January 2006 and sent back to Singapore.

He had since been held under Singapore's Internal Security Act, which allows for detention without trial.