JAKARTA (Reuters) - Indonesian police on Sunday were hunting for more suspects linked to a foiled plot to bomb targets in Jakarta, with the arrest of a woman among a trio detained a day earlier pointing to a new recruitment tactic by Islamist militants, they said.

Authorities are also concerned about smaller cells being formed by militants in Muslim-majority Indonesia, which is grappling with a resurgence in homegrown militancy, inspired in part by Islamic State.

Counter-terrorism police were looking for two more suspects after the arrests on Saturday of a woman and two men near Jakarta on suspicion of planning a bomb attack on vital national assets, national police spokesman Awi Setiyono said.

He did not elaborate, but on Saturday police said targets in Jakarta included the changing of the guard at the presidential palace.

“It is possible that their method to recruit new actors is not just limited to men,” Setiyono told a news conference, noting previously recruitment had been limited to men. “The point is they are looking for people who want to wage jihad.”

Police said they had intercepted a letter the woman had intended to send to her parents stating her intention to carry out jihad.

Later an unexploded bomb was found in a room the woman had rented in Bekasi, about an hour outside Jakarta, police said.

The suspects had been communicating with and received money from Bahrun Naim, an Indonesian militant known to be fighting with Islamic State in Syria, Setiyono said.

“On the orders of Bahrun Naim, they formed small cells and assembled bombs with other suspected militants,” he said.

Naim has been linked to a number of failed attacks in Indonesia this year, including a July suicide bombing in the Javanese city of Solo, which killed the bomber but only wounded one other person.

He was also linked to a suspected militant arrested during a raid last month where police said they found enough chemicals to build a bomb twice as powerful as the one used in the Bali bombing of 2002, which killed 202 people.

Among the chemicals found then were TNT and RDX, both military-grade explosives, and a peroxide-based explosive TATP – also known in militant circles as the “mother of satan”, which was used in the Brussels attacks this year.

Indonesia suffered its first Islamic State-linked attack earlier this year, when a gun-and-bomb assault in central Jakarta killed four people.

Security is usually stepped up in Indonesia at this time of year after attacks in previous years during Christmas and New Year celebrations.