Monaliza Romato is the niece of the clan's matriarch who was previously supporting operations. Reports have also emerged that Duterte proposed to negotiate with the group to end the siege on the city, then backed out.
The Philippines security forces arrested the main financier and logistics supporter of the pro-Daesh militant group Maute on Wednesday, the army said, as unconfirmed reports emerged that President Rodrigo Duterte had earlier planned to cut a deal with the outfit.
Security forces raided a village not far from Marawi City on Mindanao Island and detained three suspects found with ammunition and material for making bombs, army spokesman Brigadier-General Gilbert Gapay said.
The Maute group seized Marawi City in the south of the country on May 23 after the Philippines government bungled a raid to capture the so-called emir of Daesh in South Asia. Maute militants, who pledged allegiance to Daesh in 2014, are resisting daily assaults by government forces using aircraft and artillery, and with help from allies the US and Australia.
Duterte has imposed martial law in Mindanao which expires on July 23.
One of those detained on Wednesday was an important supporter of the Maute group, identified as Monaliza Romato, alias Monay. The woman is a niece of the matriarch of the Maute clan, whose menfolk lead the Marawi attack.
Two of seven Maute brothers, Omarkhayam and Abdullah, were educated in the Middle East and were the main planners of the assault on Marawi, the military has said.
"Monay has replaced her aunt as the main financier and logistic supporter of the militant group," Gapay said in a statement.
"The arrest will adversely affect the logistics support network of the group," he said.
The raid, seizure of weapons and the arrests would also prevent the militants from mounting diversionary attacks, he said.
More than 400 people have been killed in the fighting in Marawi City. Hundreds of thousands have been displaced and about 20 percent of the centre of the town has been destroyed.
Deal or no deal?
Duterte was preparing to cut a deal with pro-Daesh militants in the days after they laid siege to the southern city but aborted the plan without explanation, an intermediary involved in the process said. Duterte's security adviser and other officials denied the existence of such a deal and Reuters was unable to independently verify it.
Agakhan Sharief, a prominent Muslim cleric, said he was approached by a senior Duterte aide to use his connections with the Maute militant group's leaders to begin back-channel talks. Sharief refused to name the aide.
Sharief said the president was prepared to offer the Maute clan the implementation of Sharia law in their hometown, Butig, if he achieves his goal of establishing a federal system in the Philippines.
Two other local sources said the president had worked behind the scenes to hold talks with the Maute brothers, Omarkhayam and Abdullah.
But the process was halted when Duterte in a May 31 speech declared he "will not talk to terrorists."
The speech came five days after another televised address in which he told the militants, "we can still solve this through dialogue," but if he could not convince them of that, " ... so be it. Let's just fight."
"The problem with our president, his mind is changing always," Sharief said. "He announced he will no longer talk to terrorists and that [cut short] our negotiations."
Duterte's top peace envoy Jesus Dureza said he was unaware of any back-channel talks.
His national security adviser Hermogenes Esperon said it was unlikely Duterte had reached out to the Maute group, adding "why will he talk to the terrorists?"
However, Marawi Mayor Majul Usman Gandamra said back-channel talks did start.
He said the process failed because the rebels did not show good faith and did not reduce the intensity of attacks on government forces after Duterte offered them an olive branch.
"There was a window of opportunity," he said. "But there was no show of sincerity."
According to Sharief, Duterte's aide agreed that Sharief would accompany the Maute brothers' influential mother, Farhana, by helicopter to meet Duterte in nearby Cagayan De Oro or Davao City.
"He (Duterte's aide) prepared everything that I needed. I told him that I need a chopper to get the mother of the Maute brothers to bring her to the president. He prepared that," Sharief said.
"I called the Maute brothers and their mother ... I told them, I convinced them."
The talks with the Maute group did not go ahead and the mother was arrested on June 9 elsewhere in the same province as Marawi.
The cleric said that the rebels would have taken Duterte's deal to end the siege.
"They agreed, they supported this," said Sharief, who last met Abdullah Maute on June 25, when he led a group of emissaries into the heart of Marawi to free some hostages at the end of Ramadan.
In another part of Mindanao Island, soldiers on Tuesday found the beheaded bodies of two Vietnamese sailors abducted a year ago by militants from the Abu Sayyaf group.
At least three Vietnamese sailors are still being held by Abu Sayyaf, which, in all, holds 14 foreigners and eight Philippine citizens.
Vietnam denounced the killing of its citizens.
"Vietnam strongly condemns all savage and inhumane kidnapping and murderous acts and believes that these acts must be punished properly," Foreign Ministry spokeswoman Le Thi Thu Hang said.
The Maute group is also allied with Abu Sayyaf which is led by the emir of the Daesh in Southeast Asia.