An Alabama woman who joined the Islamic State group in Syria will not be allowed to return to the United States with her toddler son because she is not an American citizen, the US has said.

Her lawyer is challenging that claim.

In a brief statement, Secretary of State Mike Pompeo gave no details as to how the administration made their determination.

“Ms Hoda Muthana is not a US citizen and will not be admitted into the United States,” he said.

“She does not have any legal basis, no valid US passport, no right to a passport nor any visa to travel to the United States.”

But her lawyer, Hassan Shibly, insisted Ms Muthana was born in the United States and had a valid passport before she joined the Islamic State in 2014.

Islamic State Bride
Hassan Shibly, lawyer for Hoda Muthana, the Alabama woman who left home to join the Islamic State group in Syria (Chris O’Meara/AP)

He said she has renounced the terrorist group and wants to come home to protect her 18-month-old son regardless of the legal consequences.

“She’s an American. Americans break the law,” said Mr Shibly, a lawyer with the Florida chapter of the Council on American-Islamic Relations.

“When people break the law, we have a legal system to handle those kinds of situations to hold people accountable, and that’s all she’s asking for.”

Ms Muthana and her son are now in a refugee camp in Syria, along with others who fled the remnants of the Islamic State.

Mr Shibly said that the administration argues that she did not qualify for citizenship because her father was a Yemeni diplomat. But the lawyer said her father had not had diplomatic status “for months” before her birth in Hackensack, New Jersey.

President Donald Trump said later on Wednesday on Twitter that he was behind the decision, tweeting that “I have instructed Secretary of State Mike Pompeo, and he fully agrees, not to allow Hoda Muthana back into the Country!”

The announcement came a day after Britain said that it was stripping the citizenship of Shamima Begum, a 19-year-old who left the country in 2015 with two friends to join the Islamic State and recently gave birth in a refugee camp.

It also comes as the US has urged allies to back citizens who joined IS but are now in the custody of the American-backed forces fighting the remnants of the brutally extremist group that once controlled a vast area spanning parts of Syria and Iraq.

Ms Muthana’s lawyer said she was “just a stupid, naive, young dumb woman” when she became enamoured of Islamic State, believing it was an organisation that protected Muslims.

Mr Shibly said she fled her family in Alabama and made her way to Syria, where she was “brainwashed” by IS and compelled to marry one of the group’s soldiers. After he was killed, she married another, the father of her son.

After her second husband was also killed she married a third IS fighter but she “became disenchanted with the marriage” and decided to escape, the lawyer said.

US Secretary of State Mike Pompeo
US Secretary of State Mike Pompeo said the woman could not return to the US (Czarek Sokolowski/AP)

Mr Shibly, based in Tampa, Florida, said he intends to file a legal challenge to the government’s decision to deny her entry to the country.

Ms Muthana’s status had been considered by lawyers from the departments of State and Justice since her case arose, according to one US official.

The State Department declined to disclose details about her father or Ms Muthana’s case, citing privacy law.

Most people born in the United States are accorded so-called birthright citizenship, but there are exceptions.

Under the Immigration and Nationality Act, a person born in the US to an accredited foreign diplomatic officer is not subject to US law and is not automatically considered a US citizen at birth.

However, Ms Muthana’s case is unusual, if not unprecedented in that she once held a US passport.

In a handwritten letter released by Mr Shibly, Ms Muthana wrote that she made “a big mistake” by rejecting her family and friends in the United States to join the Islamic State.

“During my years in Syria I would see and experience a way of life and the terrible effects of war which changed me,” she wrote.

“To say that I regret my past words, any pain that I caused my family and any concerns I would cause my country would be hard for me to really express properly.”