Mustafa bin Sultan Ali was arrested under the Internal Security Act (ISA) last month, the Ministry of Home Affairs (MHA) said today.
He left Singapore in May and went to a regional country from where he then boarded a flight to Turkey. He had taken that travel route to hide his tracks.
He had planned to cross into Syria from the Turkish border to participate in armed violence by fighting alongside IS militants. But he was detained by the Turkish authorities and subsequently deported to Singapore last month.
Mustafa confessed that he was prepared to carry out IS-directed terrorist attacks against Western establishments in Singapore, according to a statement issued by the Ministry.
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"The Government takes a very serious view of any form of support for terrorism...And will take firm and decisive action against any person who engages in such activities," said the Ministry statement.
Earlier, a 19-year-old Singaporean M Arifil Azim Putra Norja'I was detained in May for participating in terrorism-related activities and planning to carry out violent attacks in Singapore.