JAKARTA - Indonesian President Joko Widodo may issue an emergency government regulation to deter locals from joining the Islamic State in Iraq and Syria (ISIS) amid concerns that they might pose security threat upon their return.

Counterterrorism officials fear that as many as 500 Indonesians may have joined the ISIS ranks, but the government has little ground to stop them without anti-terror laws in place, Jakarta Globe has reported.

"We are considering other forms of the law but the fastest (to be enacted) is a perppu," chief security minister Tedjo Edhy Purdijatno said refering to the Bahasa term for regulation in lieu of law, to penalize those joining or considering joining ISIS.

It is not clear what the regulation will stipulate, but the National Counter Terrorism Agency (BNPT) has been pushing for a similar emergency regulation to expand on prevailing anti-terror statutes.

The president on Thursday confirmed his administration was still looking for ways to deter Indonesians from joining the militants, Globe reported.

"But it is not yet finalized because there are pluses and minuses," Mr Joko had said.

BNPT spokesman Irfan Idris suggested that the proposed regulation could make it illegal for Indonesians to travel to "conflict prone" countries.

"If there's a group of people threatening Indonesian sovereignty and spreading hatred or rebellion, action must be immediately taken," Mr Irfan said.

Calls for the government to curtail Indonesians looking to join ISIS were renewed after 16 Indonesians were arrested by Turkish officials trying to cross over to Syria, allegedly to join the extremists. They are now being questioned. Another group of 16 who went missing last month are yet to be traced.

Indonesia has imposed a ban on citizens traveling to Syria since a rebellion against President Bashar Al-Assad ripped through the country four years ago.

"Right now there are fewer than 1,000 Indonesians in Syria but the new arrivals keep coming because of human trafficking; they're sneaking into the country using illegal agents," Mr Iqbal said.