David Cameron said Saturday the teenagers' disappearance was "deeply concerning" and underlines how every school and college needs to recognize they have a role to play in stopping people from joining what he called an "appalling death cult."
The three girls, aged 15 to 16, flew to Turkey from London on Tuesday without leaving any messages behind.
They have been identified as:- Shamima Begum, 15-years-old, possibly travelling under the name of Aklima Begum, 17-years-old.
- Kadiza Sultana, 16-years-old.
- A third 15-year-old female who is not named at the request of her family.
Police say the trio are friends with a fourth girl from their school who went to Syria in December, reported BBC News. They interviewed the three shortly after the fourth girl left Britain.
"There was nothing to suggest at the time that the girls themselves were at risk and indeed their disappearance has come as a great surprise, not least to their own families," said a statement released by the Metropolitan Police Service.
Police say they are "extremely concerned for the safety" of the girls.
"We are reaching out to the girls using the Turkish media and social media in the hope that Shamima, Kadiza and their friend hear our messages, hear our concerns for their safety and have the courage to return now, back to their families who are so worried about them."
Police said the force was becoming increasingly concerned about a growing trend of girls and young women showing an interest in joining the Islamic State group. Officials say hundreds of Britons have joined extremists in Syria.
Police say anyone with information about the girls should contact them.
'Call for women to join'
More women have been joining ISIS over the last year, and it's not surprising according to Joana Cook, a terrorism expert with King's College in London.
"Last summer, there was a call for women to join the Caliphate - an Islamic state," Cook told CBC News in an interview on Saturday. "Before, there had been no idea of a state and generally, it was a bunch of men just fighting."
Cook said ISIS has been smart in the way it recruits by showing videos of women "cooking together" and "normalizing" the idea of being part of ISIS.
"Each individual has a unique reason for joining and for women, it's generally three reasons: they see a wrong in the world they want to right or they feel oppressed or marginalized where they are [or] they see it as an individual duty."
Cook cautioned that ISIS videos do not bear out the reality for women.
"It's in fact a very harsh, dangerous existence and really hard to get out of."