MORNAG, Tunisia — In a small box in her bedroom, Oulfa Hamrounni keeps the photo she treasures most. It shows one of her daughters, brown hair flowing, a smile on her round face. The photo was taken before the girl and her sister left home to join the Islamic State's affiliate in Libya.

Today, Hamrounni is struggling to bring her teenage daughters back to Tunisia. She's also trying to prevent two others from joining them.

“I am afraid for my younger daughters,” she said. “They still have the same ideology of my older daughters.”

Her younger daughters are 11 and 13.

Hundreds of foreign female radical Islamists, including many Westerners, have journeyed to the battlegrounds of Syria and Iraq to begin new lives under the Islamic State group, also known as ISIS or ISIL. Now, there are signs that they are being encouraged to travel to Libya as well, signifying a shift in the strategy of the terrorist network as it faces growing threats and constraints to its operations in the Middle East.

Most radicalized women and girls join the Islamic State to marry fighters and bear their children, which helps the group's arm in Libya build a state, mirroring its strategy in Syria, experts who monitor jihadi activity have said.

The creation of family structures deepens the Islamic State's reach and ideology in its territory, which makes it more difficult for Western and regional governments to eradicate the militants and defuse their threat in North Africa.

“Official propaganda showcases Libya as the new frontier of the self-proclaimed caliphate,” said Melanie Smith, a researcher with the London-based Institute for Strategic Dialogue, which focuses on violent extremism. “Hence the encouragement of foreign females signifies a need to consolidate the land they have managed to acquire.”

When he announced the “caliphate” in 2014, Islamic State leader Abu Bakr al-Baghdadi specifically invited women alongside male engineers, doctors, lawyers and architects, signifying that the women's “primary responsibility is to physically build and populate territory,” Smith said.

As wives, their role is to be dutiful and obedient to their militant husbands. As mothers, they nurture the next generation of fighters. Some women also have combat duties.

Rahma Hamrounni, 17, became the wife of Noureddine Chouchane, a senior Tunisian Islamic State commander who is thought to have been killed in a U.S. airstrike on the Libyan city of Sabratha on Feb 19. Her 18-year-old sister, Ghofran, was married to a militant who was killed after the attack. Six months ago, she gave birth.

Both sisters are now in the custody of an anti-Islamic State militia in Tripoli, the Libyan capital.

More than 700 Tunisian women have joined the Islamic State and other militant groups in Syria and Iraq, according to the nation's Ministry of Women. Badra Gaaloul, a researcher with the Tunis-based International Center of Strategic, Security and Military Studies, estimates there are over 1,000 female foreign radical Islamists in Libya, including 300 Tunisians.

“They serve as wives, mothers, as religious instructors to teach the laws of the Islamic State,” Gaaloul said. “They also police areas and train to be fighters and suicide bombers.”

Researchers are noticing efforts on social media to lure more female radical Islamists to the Libyan coastal city of Sirte, which the Islamic State seized in the chaos after the death of Libyan dictator Moammar Gadhafi five years ago. In tweets, monitored last fall by the Institute for Strategic Dialogue, female radical Islamists urged followers to head to Libya, noting that routes from Turkey into Syria were blocked.

By 2014, Rahma and Ghofran Hamrounni were attending ceremonies celebrating the martyrdom of Tunisian Islamist extremists killed in Syria. Through social media and websites, they learned about the armed groups fighting there. They placed the black Islamic State flags in their bedrooms.

“By then, I had lost control of my daughters,” Oulfa Hamrounni said.

They also began to radicalize their younger sisters, Taysin and Aya. They bought a toy Kalashnikov rifle and showed them how to operate it. They showed them videos of how the Islamic State trained children to use weapons.

“We used to watch how they taught children to become snipers,” said 11-year-old Taysin.

“They always told me to join ISIS and go into the field and fight,” said 13-year-old Aya.

In late 2014, Hamrounni crossed the border with her family to the Libyan city of Zawiya to find work. The war's violence had not reached there.

Within weeks, Ghofran had fled the house. Two days later, the family returned to Tunisia. Hamrounni restricted Rahma's movements, but it didn't stop her aspirations.

Last summer, she also vanished.

In Libya, while her sister was a dutiful wife of a militant, Rahma trained in weapons. Her mother thinks she was in Sabratha with other Tunisian Islamist extremists to launch an attack in Tunisia. After the U.S. airstrike, the sisters were captured.

In a phone interview, Ahmed Omran, a spokesman for the Libyan militia, acknowledged that the girls were in their custody but declined to comment further.

Hamrounni has gone on national television, chastising the Tunisian government for not doing more to get her daughters released, even though she is aware they will be thrown in jail. Hamrounni no longer allows her two younger daughters to access Facebook. She doesn't let them speak to their older sisters the rare times they call.

“I am not with the Islamic State now,” said Taysin, a precocious girl dressed in pink with a black headscarf.

But as the conversation flowed, it became apparent that she still felt some sympathy for the militants' ideology.

“The nonbelievers, they have to be killed,” Taysin said. “The nonbelievers are trying to beat Islam. We have to fight them.”

When asked how she felt about her older sisters joining the Islamic State in Libya, she replied:

“They did the right thing.”