Indonesian militant cells are now recruiting women to mount terror attacks, said national police chief Tito Karnavian.
The top cop's warning came after anti-terror police commandos arrested a fourth woman this week on suspicion of terrorism.
Investigations are under way to determine if she is linked to a new terrorist cell behind a plot to bomb the presidential palace in Jakarta last Sunday.
"Yes, it is a new mode of operation," said General Tito, when asked about the threat that has emerged in recent days.
He said that militants make use of women to carry out attacks because, unlike men, they appear less suspicious. "Indeed, there was a plot to use a 'rice-cooker' bomb (for an attack), and the bomb was to be detonated by a woman."
His comments followed reports that a 34-year-old Indonesian woman was nabbed by the Detachment 88 (Densus 88) counter- terrorism unit on Thursday in Purworejo, Central Java.
Purworejo police chief Satrio Wibowo told the Antara state news agency that Ika Puspitasari was arrested at a mosque near her home.
"The suspected terrorist and evidence have been sent to Jakarta for further investigations," he said, adding that Densus 88 officers had also seized a passport, mobile phone and books from her home.
According to local news reports, Ika had returned to Purworejo about two months ago.
Her family told Radar Jogja Online, a local news portal, that she had worked as a maid in Malaysia and was planning to be a housekeeper in Hong Kong.
Over the past week, Densus 88 commandos have arrested at least 10 members of the cell based in Solo, Central Java.
Police said the cell was set up by Bahrun Naim, an Indonesian militant who is in the Middle East fighting alongside the Islamic State in Iraq and Syria (ISIS) militant group.
Among the members of the group arrested so far are three women, including Dian Yulia Novi, who had worked as a maid in Singapore and Taiwan. Dian, 27, had hidden a homemade "rice-cooker" bomb in her rented room where she was arrested last Saturday in Bekasi, West Java.
The other two women are Arida Putri Maharani, 25, who was arrested in Solo last Sunday, and Tutin Sugiarti, who is 37. Tutin, who was nabbed in Tasikmalaya, West Java, on Thursday, is said to have played a part in recruiting Dian for the cell.
Only Dian and Arida were preparing to mount suicide bombings. Both are married to Muhammad Nur Solikin, the 26-year-old leader of the terror cell, who is among the men rounded up in connection with the plot to attack Istana Merdeka.
Jakarta-based Institute for Policy Analysis of Conflict (Ipac) director Sidney Jones told The Jakarta Post that online chatter indicates that some of these women have expressed admiration for other women who played active roles in the terrorist movements in Europe.
"While some (Indonesian) women are content to just be wives and mothers, they want more active roles in fighting," she said.
