Shamima Begum was 15 when she ran away to join ISIS with two school friends in 2015 (Reuters File Photo)

A London teenager who joined ISIS in Syria called on the British authorities on Thursday to reconsider their decision to revoke her citizenship, calling for "mercy".

"I would like them to re-evaluate my case with a bit more mercy in their heart," Shamima Begum told Sky News from a refugee camp in eastern Syria.

Begum was 15 when she ran away to join ISIS with two school friends from Bethnal Green in east London in 2015.

Now a 19-year-old mother, she has become a refugee after the group's proto-state collapsed.

Separated from her Dutch ISIS fighter husband, and after giving birth to a son in the camp last weekend, she now wants to come home.

Her family's lawyer, Tasnime Akunjee, is looking at challenging Britain's decision to revoke her citizenship, which risks leaving Begum stateless.

The Guardian newspaper reported that he plans to travel to the camp to seek Begum's consent to bring her newborn son back to Britain while the case plays out.

"I would hope that I would be able to outline the options for her, explain things to her. We would want her agreement and consent of course," he said.

But Begum told Sky News she would not allow her son -- her third child, after the other two died in recent months while living under ISIS -- to leave Syria without her.

Asked if she can change or be rehabilitated, she replied: "I am willing to change."

Begum's family are from Bangladesh but the country said Wednesday she had no claim to citizenship there.