The UK is trying very hard to prevent Shamima Begum, the British teen who joined ISIS four years ago, from returning home.

It started the process of revoking her citizenship earlier this week, but it's a complicated process as international law prevents a nation from leaving someone stateless.

Begum's family has told the UK government that it will launch a legal appeal the decision to strip her of citizenship.

The family of Shamima Begum, the teenager who fled London to join ISIS in Syria, say they want to appeal the UK's decision to strip her of citizenship, which could result in Britain being forced to accept her back.

Begum, now 19, fled her home in east London for Syria to marry an ISIS fighter alongside two school friends in 2015.

She escaped Baghouz, the village considered the last bastion of ISIS's waning caliphate, while nine months pregnant about three weeks ago in order to save her then-unborn child.

She is now trying to return home with her newborn son, Jerah, to whom she gave birth in a Syrian refugee camp last weekend.

But her fight to return home is not easy. The UK considers her a national security threat and have worked to prevent her return.

Read more: A British teen who fled with friends to join ISIS four years ago is now pregnant and living in a Syrian refugee camp. Here's how her journey unfolded.

Shamima Begum swaddles a baby, likely her newborn son Jerah, during an interview with ITV News published on February 20, 2019. ITV News/YouTube

Earlier this week the British Home Office told Begum's parents via letter that it had started the process of taking away her British citizenship.

Her family has since challenged the decision and threatened in a letter to Sajid Javid, the British Home Secretary, that they would appeal his decision in court.

Renu Begum, Shamima's sister, said in the Thursday letter published by the BBC that her family was "sickened" by Shamima's recent comments to the press, but suggested that it was because her sister was "groomed" by ISIS.

Shamima has told British media over the past weeks that she doesn't regret joining ISIS, though she says she no longer agrees with their beliefs.

She also told The Times of London that seeing a severed head for the first time "didn't faze me at all" because it was from "an enemy of Islam," a term that ISIS converts referred to people fighting the group.

A woman walks past vehicles near Baghouz, Deir Al Zor province, Syria, on February 12, 2019. Rodi Said/Reuters

Renu wrote:

"My sister has been in their [ISIS's] thrall now for four years, and it is clear to me that her exploitation at their hands has fundamentally damaged her.

"I have watched Shamima on our televisions open her mouth and set fire to our nation's emotions.

"As we have already expressed, we are sickened by the comments she has made, but, as a family man yourself, we hope you will understand that we, as her family cannot simply abandon her.

"We have a duty to her, and a duty to hope that as she was groomed into what she has become, she can equally be helped back into the sister I knew, and daughter my parents bore.

"We hope you understand our position in this respect and why we must, therefore, assist Shamima in challenging your decision to take away the one thing that is her only hope at rehabilitation, her British citizenship.

"Shamima's status will now be a matter for our British courts to decide in due course."

Renu added that Shamima's son "is the one true innocent and should not lose the privilege of being raised in the safety of this country."

Javid hinted on Wednesday that he would not remove the baby's rights to a British citizenship, The Guardian reported.

Read the full letter here.

A still from a February 20, 2019 video showing Shamima Begum, holding her son, reads the Home Office's letter to her mother which said that it was revoking her citizenship. ITV News/YouTube

Stalemate

Revoking Begum's British nationality is not a simple process for the government either.

Under UK law, the government cannot take away someone's citizenship unless they are also a citizen of another country. British officials reportedly told Begum's parents that because they are of Bangladeshi heritage, their daughter might be able to apply to be a Bangladesh citizen.

Bangladesh's government responded to the news by saying that Begum was not a citizen and refusing to take her in.

Begum said earlier this week she could try to move to the Netherlands, the home country of her husband — a Dutch Islam convert and ISIS fighter — when she learned that her country would rescind her citizenship.

Shamima Begum in an interview with ITV News on February 20, 2019. ITV News/YouTube

A spokesman for the Dutch Ministry of Justice and Security declined to comment on individual cases, but told INSIDER in a statement that to live in the Netherlands, a person would need a residence permit. Begum does not have this.

The spokesman added that the country does not offer any help to foreign fighters trying to return home, and would arrest and prosecute them if they got back.

Read more: Despite trying to strip her of citizenship, the UK could be forced to take back ISIS teen bride Shamima Begum after all

Women sit with their belongings near Baghouz, Syria, on February 12, 2019. Rodi Said/Reuters

What next for Shamima?

Tasnime Akunjee, the Begum family's lawyer, said he or another lay wer plans to go to the Syrian refugee camp where Begum is staying so she can appoint legal representation to start her process to go home, Sky News reported.

He added that having a British baby with a stateless mother — as would be the case if Begum is stripped of her citizenship but her baby is not — was "a bizarre scenario," according to Sky News.

Begum's struggle to return home comes as several women who fled their home countries to join ISIS — so-called "ISIS brides" — escaped the caliphate and are now asking to return home.