Two men -- Mohammad Razif Yahya, 27, and Amiruddin Sawir, 53 -- were arrested in August 2015 and held under the ISA, which allows for detention without trial, the Ministry of Home Affairs (MHA) said.
Both men had gone to Yemen at different times for religious studies but they also took part in the country's armed sectarian conflict.
MHA said the men volunteered for armed sentry duties at the religious institution they were studying at and were involved in gun battles against Yemen's Houthi rebels.
"By taking up arms in Yemen, they have demonstrated a readiness to use violence to pursue their religious cause. As such, they are assessed to pose a security threat to Singapore."
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A third man, Mohamed Mohideen Mohamed Jais was in Yemen from 2009 to early 2011 for religious studies. During his time there, Mohideen performed armed sentry duty but is believed not to have been involved in firefights.
He has been issued a restriction order, which means his movements will be monitored and he will be made to attend compulsory religious counselling sessions.
In January, he travelled to an unnamed country from where he intended to make his way to Turkey and then Syria but was caught and sent back to Singapore.
MHA said Wang brought along his combat boots and uniform issued by the Singapore Armed Forces during mandatory conscription for able-bodied male citizens.
MHA said even though Wang's action was not ideologically-driven as he had intended to fight against the IS group, he still wanted to engage in an armed conflict overseas.