Singapore arrested four citizens who travelled overseas to take part in armed sectarian conflicts in the Middle East.
The arrested men include three radicalised Muslim youth who fought in Yemen and a man of Chinese descent who travelled overseas to fight the Islamic state on the side of Kurdish militia groups.
The government said on Wednesday the four men were arrested in separate cases under the country's Internal Security Act (ISA).
The Ministry of Home Affairs said two of them have been released after investigations, with strict restrictions placed on their movements. The Internal Security Act allows for indefinite detention of suspects without charges.
The ministry said Mohammad Razif Yahya and Amiruddin Sawir are still kept in detention for "voluntarily taking up arms and participating in the armed sectarian conflict in Yemen."
Officials said the radicalised youth took up religious studies in Yemeni institution in 2010 and 2013 and joined the fight against the Houthi rebels in the country.
They have been held without trial since August last year. "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," the ministry said.
"Razif and Amiruddin were prepared to kill and be killed as 'martyrs' in the sectarian conflict in Yemen," the MHA statement added.
A third youth, Mohamed Mohideen Mohamed Jais, who participated in the fight against the Houthis alongside the other two, was held under restriction orders as it was found that he did not fire weapons but was on duty as an armed sentry.
Wang Yuandongyi, an ethnic Chinese who underwent the National Service and took up citizenship in 2014, plotted to join the Kurdish militia fighting the Isis but was detained in a foreign country and sent back to Singapore.
Wang is a non-Muslim and the authorities do not believe he was religiously indoctrinated.
"Even though his motivation to join the Kurdish militia group and fight against IS in Syria was not ideologically driven, the fact remains that he intended to engage in an armed conflict overseas," MHA said.