KUALA LUMPUR - Police in Malaysia said yesterday they are hunting the accomplices of 12 Malaysians detained on suspicion of planning attacks on Putrajaya and other key targets, including the Kuala Lumpur court complex.

Home Minister Ahmad Zahid Hamidi said efforts were being made to track down the suspected members of the Islamic State in Iraq and Syria (ISIS) militant group.

"We are very serious in tracking them down... I'm sure the police will be able to find them wherever they are. Apart from monitoring airports, the police are also monitoring land and sea routes, and their activities and communications on social media," he told reporters yesterday.

It was not clear how many accomplices the police were looking for.

The 12 men, aged between 17 and 41, were arrested by the Special Branch Counter-Terrorism Division in an operation in Kuala Lumpur over the weekend.

The Malaysian Insider reported yesterday that among the suspected targets were the area where the Kuala Lumpur court complex is located, and Bukit Aman, site of the police headquarters.

Among the items seized from the suspects were 20kg of ammonium nitrate, a similar amount of potassium nitrate, an ISIS flag, about two litres of liquid believed to be kerosene, two units of remote controls, three PVC pipes and equipment believed to be used in producing explosives.

Datuk Seri Zahid said that although the group's members had never been to Syria or Iraq, the plot showed they had a network and were ready to commit acts of terror.

He added that the group was not only preparing explosives but also carrying out tests, which, if not prevented, would bring about bigger threats to the targets.

The arrests came as Malaysia hosted a summit of the 10-member Asean that ended yesterday.

The suspects were planning revenge attacks after some of their members were arrested by police, national police chief Khalid Abu Bakar said at the weekend, although he made no direct link between the alleged plot and the summit.

Malaysian police say dozens of the country's citizens have gone to Syria to join ISIS, and have warned of radicals returning to stage attacks.

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

The Malaysian Insider quoted sources as saying police picked up 11 of the suspects while they were on their way up Gunung Nuang in Hulu Langat last Saturday evening. "They were on their way up to the mountains to test the homemade bombs," one source said.

The last suspect was detained in a follow-up operation the next day.

Methods to launch the planned attacks were varied, said sources.

Police found bomb-making materials in the suspects' backpacks, and the authorities did not discount the possibility of the group intending to use suicide bombers.

"It can happen. There are so many possibilities. They can put (the bombs) under a car, or use suicide bombers... anything," The Malaysia Insider quoted one of the sources as saying, on condition of anonymity.

The sources said police are investigating how the group came together, as checks revealed they did not know one another prior to the "mountain-trekking activity".