Months after declaring an Islamic caliphate, Islamic State, which has seized large swaths of Syria and Iraq, is seeking to address a need of any viable nation: women.

In Internet posts and social media messaging, the extremist Sunni militants are recruiting women to marry their fighters and have children, part of a larger strategy of state-building.

"They are treating the Islamic State as a country that needs women," said Rita Katz, director of the SITE Intelligence Group, which monitors online activity by militant organizations. "The message is: 'You are coming to marry someone immediately and have kids and cook.' They're building a state."

As the Al Qaeda breakaway group comes under attack by the United States and other nations, and risks losing territory it has captured, a dual-gender expansion of membership could make it—or at least its ideology—more difficult to dislodge.

Even before the emergence of Islamic State, small numbers of women—mostly Europeans—began arriving in parts of Syria as wives of foreign fighters, which "encouraged the original image of Syria being a five-star jihad," said Charles Lister, a visiting fellow at the Brookings Doha Center, a Qatar-based think tank.

"The fact that entire families have been arriving in northern Syria for well over a year now has undoubtedly served to entrench the perception that a state or at least an identifiable new Islamic creation is forming," Lister said.

Using the social media tools that have helped attract thousands of Islamic militants from all over the world, members of the group are encouraging women to journey to Syria to start families.

The extremist group Islamic State has seized large swaths of territory in Iraq and Syria for its self-styled caliphate. The militants espouse an especially harsh interpretation of Islam and have used brutal tactics to impose their rule.

Female recruits who have made the trek are chiming in: On sites like Twitter, Tumblr and ask.fm, they use the abbreviated and grammar-challenged writing characteristic of the Internet, employing lots of LOLs and emoticons even as they advocate attacks on religious minorities, quote extremist religious leaders and cheer the beheading of American journalists.

One online poster, Umm Ubaydah, claims to have traveled alone to Syria to take care of her husband and fellow Islamic State fighters. Three days after American reporter Steven Sotloff was executed, one person asked on her Tumblr page, where she occasionally writes in Swedish, what she thought of his execution. "I wish I did it," she answered simply.

Dispatches out of Islamic State paint a picture of an emerging nation with its own government institutions, school curricula and traffic laws. There are occasional suggestions that female recruits can become teachers, nurses or doctors. But some residents have reported that women living in Islamic State-controlled areas are not allowed to leave the house without a male chaperon, risking detention or even whipping if caught.

Six, seven months ago you had maybe one or two, but suddenly you have so many popping up. — Rita Katz, director of the SITE Intelligence Group, saying more women are responding to Islamic State's call

Reports of women answering the call to support the Islamic State are on the rise. "Six, seven months ago you had maybe one or two, but suddenly you have so many popping up," Katz said.

This month, a 19-year-old Colorado woman pleaded guilty in a Denver federal court to providing material support to a foreign terrorist organization after the FBI arrested her in April as she attempted to travel to Syria. At least one Minnesota woman is believed by authorities to have traveled to Syria in recent weeks—telling her parents she had gone to help treat the wounded—and the FBI is looking into additional reports.

Over the Labor Day weekend, a teenage girl planning to go to Syria was arrested at an airport in southern France, and the man who allegedly recruited her was arrested shortly afterward. Days later, the family of a 20-year-old Scottish woman revealed that she had traveled to Syria in November 2013, where she is believed to have been posting online under the pseudonym Umm Layth and encouraging other young women to make the same journey.

One Islamic State fighter recently wrote on his Tumblr page that "on average, around 10-20 sisters, sometimes more, arrive here in Islamic State [territory] everyday."

Though official Islamic State channels haven't made specific calls for women, leader Abu Bakr Baghdadi has urged all Muslims to make the trip to Syria, maintaining that "immigration to the land of Islam is obligatory."

Seeking to promote the Islamic State as more than a group of warriors, he called upon scholars, judges, "as well as people with military, administrative and service expertise and doctors and engineers of all different specializations and fields," according to a speech released online.

The recruitment of women defies the conservative Muslim belief that a woman needs permission from her male guardian to travel or marry. Recruiters insist that the obligation to live in the caliphate trumps the requirement for permission.

Online accounts post tips on skirting Turkish security to slip through the border with Syria and enter Islamic State territory.

To avoid detection, Islamic State recruits are advised to book a round-trip ticket to Turkey, apply for a tourist visa and research sightseeing attractions in case they are questioned by customs officials or police. One Islamic militant directed women to pack an extra abaya, a black robe, in case the first one is torn crawling under the barbed-wire fence along the border.

Umm Ubaydah, the online poster, also fields more mundane questions about travel to a war-torn country.

"Can one find good hair dryer and straighter there and hows the weather like, I have no winter cloth with me," one anonymous poster asked. There are also queries about the availability of coffee, tea and medicine, "changes in your health, hair, skin and eyes lol," and which smartphone applications to download.

Beyond marrying militants and having children, there are mixed messages about the role of women in an Islamic caliphate.

An unofficial Islamic State media group, apparently run by female militants, reactivated its Twitter account Sept. 4, according to SITE. Though the Khansa Media Battalion's Twitter account was suspended just one day later, it was up long enough to post a video in which women in niqabs, or face veils, were shown firing rifles in a desert and other armed women were making threats against Syrian President Bashar Assad, former Iraqi Prime Minister Nouri Maliki, Iran and Russia.