The Malaysian authorities have arrested six people suspected of links to the Islamic State in Iraq and Syria (ISIS) in a month-long operation, police said.

The suspects - four men and two women - were nabbed in Kelantan, Pahang, Malacca, Johor and Penang, said a statement. Two men are suspected of smuggling arms from southern Thailand.

One woman, 26, was detained by the Turkish authorities in February while allegedly waiting for instructions from Malaysian terrorist Muhammad Wanndy Mohamed Jedi to enter Syria. The other woman allegedly promoted ISIS through the Google+ app.

As for the remaining two men, one had returned from taking part in militant activities in Syria, while the other allegedly used 15 Facebook accounts to propagate the ISIS agenda and had plans to attack Shi'ite mosques in Penang.

Police are still looking for a third arms smuggling suspect - 27-year- old Muhammad Muzaffa Arieff Junaidi from Kelantan, who is believed to have fled to Thailand.

"The suspect is believed to have illegally entered southern Thailand through Golok river on March 22, 2017, with two weapons - an M4 carbine and a pistol," Inspector- General of Police Khalid Abu Bakar said in the statement.

Between 2013 and the end of last year, the Malaysian authorities arrested more than 250 ISIS suspects, most of them Malaysians. Police have charged more than 90, with at least 48 convicted and jailed.

Tan Sri Khalid said the authorities deemed reports of Muhammad Wanndy's death "suspicious". He was allegedly killed in a drone attack in Syria last Saturday, according to a Facebook posting by his wife, who followed him to Syria in January last year.

"It is deemed suspicious as there are other reasons that may have led him to stage his death," the New Straits Times quoted the police chief as telling reporters.

He said that though Muhammad Wanndy's family in Malacca had also received news of his death, "we still have our doubts".

A group acting on Muhammad Wanndy's orders claimed responsibility for a grenade attack on a bar last June in Puchong, Selangor, that injured eight people.