Malaysia has revoked the passports of 68 citizens who went abroad and are believed to have links to the Islamic State in Iraq and Syria (ISIS) militant group, Deputy Prime Minister Zahid Hamidi has said.

"They no longer have travel documents as Malaysian citizens. If they return, legal action in line with the Immigration Act will be taken against them," the Bernama news agency quoted Datuk Seri Zahid as saying on Sunday.

A total of 73 Malaysians travelled to join ISIS, according to previous news reports. Nineteen were killed while fighting and eight have returned to Malaysia. This leaves 46 still believed to be in Syria.

Among those who went to Syria were two families with young children who had sold their properties and assets to join ISIS.

"These two families left everything and brought along their young children to Syria, purportedly to fight for jihad (holy war) as a shortcut to heaven," Mr Zahid said.

The authorities have cracked down on local militant activities and arrested ISIS suspects since 2014. Police have so far arrested more than 200 suspected militants, of whom at least 12 have been charged.

Most of the Malaysians who joined ISIS are reported to have been given menial tasks such as cleaning or cooking.

But with ISIS losing ground in Syria, it is looking to gain support abroad. In June, it uploaded a video showing Malaysian ISIS member Mohd Rafi Udin threatening attacks against Malaysian police and decapitating a captive.

Malaysia experienced its first terrorist attack five days later when local ISIS cell members threw an old hand grenade into a suburban pub, injuring eight people. The Straits Times understands that the grenade had been smuggled in from a neighbouring country.

Mr Zahid said the authorities are tracking another eight hand grenades they believe are in ISIS possession in Malaysia. Illicit firearms, mostly from Thailand, Indonesia and the southern Philippines, are often brought into the country via its porous borders.