Singapore's government announced Wednesday that it has arrested four citizens accused of links to armed conflict in Yemen and to a Kurdish militia group fighting against the Islamic State group.

It said the four men were arrested in separate cases under the country's Internal Security Act, which allows indefinite detention without charge. Two have since been released with restrictions on their movements and speech, the Ministry of Home Affairs said.

The arrests came as Singapore authorities have urged citizens to be more vigilant in dealing with the threat of terrorism. Singapore is a multiethnic nation, and its leaders have repeatedly warned against the dangers of religious extremism, saying it could permanently tear the small city-state's ethnic fabric.

The ministry said two of the men, Mohammad Razif Yahya and Amiruddin Sawir, have been held without trial since last August for "voluntarily taking up arms and participating in the armed sectarian conflict in Yemen." It said both men underwent religious studies in Yemen and fired weapons such as AK-47 rifles in fighting the Houthis.

"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 in a statement.

It said two other men, Mohamed Mohideen Mohamed Jais and Wang Yuandongyi, were released this month under orders that ban overseas travel, public statements and joining any organization without approval. It did not say when the two were arrested.

Jais performed armed sentry duties in Yemen but did not open fire, the ministry said.

Wang had intended to join a Kurdish militia group fighting the Islamic State group and left Singapore in January for Syria, but was caught in a transiting country, it said.

"Even though his motivation . was not ideologically driven, the fact remains that he intended to engage in an armed conflict overseas," it added.

In February, Singapore deported four Indonesians allegedly on their way to Syria to join the Islamic State group.