A 15-year-old girl was arrested by British police Thursday on suspicion of plotting to join the Islamic State.

Authorities said the girl, who was arrested Thursday morning in east London, was placed in custody at a central London police station as she awaits further questioning.

The announcement of Thursday's arrest comes the same day British police charged a 26-year-old woman with being a member of ISIS and inciting terrorism on social media, according to Reuters.

The woman, who has not been named, was arrested by counter-terrorism officers at London's Heathrow Airport in February after arriving on a flight from Turkey. She has been charged with publishing messages on Twitter in October 2014, encouraging acts of terrorism, and being a member of the terrorist organization.

Authorities in the UK remain on high alert following estimates by intelligence officials that hundreds of British citizens have joined the Islamic State in Iraq and Syria. Earlier this year, three British schoolgirls, Shamima Begum and Amira Abase, both 15, and Kadiza Sultana, 16, fled their east London homes to join the terrorist network in Syria.