LONDON (AFP) - A mother who took her toddler to Syria and joined the Islamic State in Iraq and Syria (ISIS) group is thought to have become on Friday the first British woman to be convicted after returning home.

Tareena Shakil, 26, was found guilty of ISIS membership and encouraging terrorism in posts on Twitter before leaving Britain.

The court in Birmingham, central England, heard she was radicalised on the Internet and in October 2014 told her family she was going to Turkey for a beach holiday.

Instead, she crossed the border into Syria and then went to ISIS stronghold of Raqqa.

"I left to build us all a house in heaven, allah promised us heaven if we sacrifice our worldly life x," she wrote in a message to a relative. "I'm not coming back."

In Raqqa, she was kept in a large house with other single women and posed with her son for a selfie while wearing a black balaclava branded with the ISIS symbol.

Other pictures found on her phone showed her posing with an AK-47 assault rifle and a hand gun.

However, Shakil found life under ISIS rules too strict.

In January 2015, after repeatedly looking up "I want to leave ISIS" on the Internet, she and her son travelled by road to the Turkish border.

They ran 1km to escape into Turkey, dodging a three-man ISIS patrol before handing themselves into the Turkish military, she told the court.

She was arrested when police boarded her flight home at London's Heathrow Airport last February.

During her trial, Shakil claimed she only travelled to Syria because she wanted to live under sharia law.

She will be sentenced on Monday.