The alleged leader of an Islamic militant group accused of plotting to crash an airliner in Singapore has been arrested in Malaysia after more than a year on the run.

Prime Minister Najib Razak said Mas Selamat Kastari, who escaped from a high-security detention centre in Singapore by crawling through a toilet window, was plotting new attacks on the city-state when he was detained.

"We apprehended him here, his main focus at the time was Singapore," he told reporters. "He was planning a lot of things in Singapore."

"I congratulate the police on arresting someone who is deemed able to pose a threat to security," he said.

Home Minister Hishammuddin Hussein said Mas Selamat was being held under internal security laws but would not confirm reports he was arrested on April 1 in Johor, which is separated from Singapore by a narrow waterway.

mas selamat alleged ji terrorist captured by malaysia authorities 080509 04 "We are becoming an expert on him so hopefully this time he won't escape us, and the Singaporean experience will help," he said of the Indonesian-born militant who has escaped custody several times.

Police chief Musa Hasan said he was detained early last month along with two other suspected militants in a joint operation by Singapore and Malaysian police.

"We are in contact with our counterparts (in Singapore and Indonesia) and have informed them about what we have gathered from them," he told AFP .

Mas Selamat is said to be the head of the Singapore cell of Jemaah Islamiah (JI), an underground group linked to Al-Qaeda and blamed for the 2002 Bali bombing and other bloody attacks in Southeast Asia.

mas selamat alleged ji terrorist captured by malaysia authorities 080509 03 Singapore officials alleged he was part of a plot to hijack an airliner in Bangkok and crash it into Changi airport - one of Asia's busiest - in 2001 following the September 11 attacks that year in the United States.

Now 48, he escaped from his high-security detention centre in Singapore on February 27 last year after squeezing through a toilet window that had no bars and climbing over a fence.

His flight triggered a huge manhunt, but a flood of tips from the public, some inspired by a bounty of S$1 million (US$647,520) put up by two local businessmen, turned out to be false alarms.

The affair made the strict city-state an object of ridicule and triggered a sweeping review of security measures.

Mas Selamat swam across Johor Strait

Najib said he had managed to enter Malaysia a few days after leaving the detention facility.

Singapore Home Affairs Minister Wong Kan Seng told broadcaster Channel NewsAsia that Mas Selamat swam across the Johor Strait that separates the two countries, using an improvised flotation device.

The militant had not been formally charged at the time of his escape, and was being held under Singapore's Internal Security Act which - like the Malaysian equivalent - allows for detention without trial.

He had fled Singapore in December 2001 after a security operation against Jemaah Islamiyah but was arrested in Indonesia in 2006 and handed back.

"He is an extremely skilled and dangerous terrorist and the fact that he has been recaptured improves the security situation in Singapore and the region," said John Harrison, a security analyst at Singapore's Nanyang Technological University (NTU).

"This is irrespective of what he may or may not have been able to accomplish during his escape," he told AFP .

Harrison praised Singapore and Malaysian authorities for their cooperation and said they had kept the news under wraps to get more intelligence away from the media glare.

Sidney Jones, an analyst with the International Crisis Group, applauded the arrest, but said it did not erase the threat.

mas selamat alleged ji terrorist captured by malaysia authorities 080509 05 "In some ways the bigger danger will still come from the people at large," she told AFP from Jakarta.

"I think we've got a number of little splinters. I don't think this arrest will change their strategies."

Rohan Gunaratna, a security analyst at the S Rajaratnam School of International Studies in Singapore, described Mas Selamat's arrest as a "very significant blow to the JI network because JI is trying to revive."

"Mas Selamat is a key icon of the Jemaah Islamiah organisation, he is one of the most dangerous terrorists in this region, his arrest is a major setback to the JI organisation," he told broadcaster Channel NewsAsia.