Malaysian police said they arrested on Sunday 12 suspected Islamic militants who were plotting attacks in the country just as it prepares to host a Southeast Asian summit.

   National police chief Khalid Abu Bakar said the suspects, arrested in separate raids in suburbs of the capital Kuala Lumpur, were all Malaysian men and suspected supporters of the extremist Islamic State (IS) group.

   Police also seized 20 kilograms (44 pounds) of ammonium nitrate, a similar amount of potassium nitrate, two litres of kerosene and remote-control devices, he said in a statement.

   "They had planned to launch attacks in Malaysia on April 25 and April 26," Khalid said.

   They were targeting "strategic" locations in and around Kuala Lumpur, Khalid said, but he gave no details.

   Muslim-majority Malaysia hosts Monday's summit of the 10-member Association of Southeast Asian Nations (ASEAN), which will begin the day in the capital before shifting to the resort island of Langkawi.

   Malaysia has said it plans to call at the gathering for greater regional cooperation against extremism.

   The militants were planning revenge attacks after some of their members were arrested by police, Khalid said. His statement made no direct link between the plot and the summit. 

   Malaysian police say dozens of its citizens have gone to Syria to join the brutal IS jihad, and have warned of radicals returning to stage attacks on home soil.

   Over the past year, police have announced a series of arrests of suspects they say were IS sympathisers plotting such attacks.

   But opposition lawmakers say authorities have shared no details with them on the dozens of claimed arrests or the extent of the purported terror threat.

   Some lawmakers have expressed scepticism over the police claims, accusing authorities of hyping the threat to justify passage of a tough anti-terrorism law earlier this month, and a broader crackdown on civil liberties over the past year.

   A day before the terrorism law passed, police announced the arrests of 17 Islamic militants they say were plotting attacks.

   The anti-terror legislation allows suspects to be held virtually indefinitely without judicial review, drawing fire from the parliamentary opposition and human rights groups who say the government has a history of using security law against critics. (AFP)