"Officers are working closely with the Turkish authorities who are providing a great deal of assistance and support to our investigation,” a Scotland Yard spokesman told the BBC.
The three girls -- Shamima Begum, 15, and her friends Amira Abase, 15, and Kadiza Sultana, 16 -- were all last seen Tuesday morning when they boarded a flight from London's Gatwick airport headed for Turkey. From there, police believe they made their way to Syria to join the Islamic fighters.
While their classmates head back to school at Bethnal Green Academy after a week-long vacation, the school's principal worked to reassure other parents that the school was handling the situation.
"We are all shocked and deeply saddened by the news that three of our students have been reported missing from home," school principal Mark Keary said to the BBC.
"This situation follows an earlier disappearance of a student in December of last year. The police spoke to the student's friends at that time and, further to this, they indicated that there was no evidence that the girls were at risk of being radicalized or absconding."
The girls' relatives have been making emotional pleas to get them home.
"If you're watching this, baby, please come home," said Renu Begum, an older sister of one of the missing girls, Shamima Begum.
"Mum needs you more than anything in the world. You’re our baby and we just want you home, we want you safe. Just contact anybody let them know that you need help," Renu Begum told the BBC.
"You've got all the help in the world you're not in any trouble here, we all love you. If anybody's convinced you of anything then they're wrong. We love you more than anybody that could ever love you," Begum said.
Renu Begum remembered asking her sister about that at the time, saying, "You wouldn't do something stupid like that would you?"
Abase Hussen, the father of missing girl Amira Abase, told the BBC his daughter told him she was going to a wedding when she left the family home.
A relative of the third missing girl, Kadiza Sultana, said "everyone's hurting... especially mum."
"Find the courage in your heart to contact us and let us know that you're safe and you're OK," the relative said.
ABC News' Kirit Radia contributed to this report.