UK police said 15-year-old Shamima Begum and two other London girls took a flight to Istanbul from London Gatwick Airport on Tuesday.
Their families have reported them missing, and London's Metropolitan Police say the girls are believed to be heading to Syria, where militant groups and government forces are locked in a war, and parts of which have been taken over by the Islamist terror group ISIS.
"We miss you terribly and are extremely worried about you," Shamima's family said in a statement that Metropolitan Police released Saturday. "Please, if you hear this message, get in touch and let us know you are safe. We want you home with us. You belong at home with us.
"Syria is a dangerous place and we don't want you to go there. Get in touch with the police and they will help to bring you home. You are not in any trouble."
The family said it understands that Shamima has strong feelings about wanting to help those she believes are suffering in Syria.
"You can help from home, you don't have to put yourself in danger," the statement reads. "Please don't cross the border. Please come home to us. Our Mum needs you home and is really worried. We are not mad at you, we love you."
Police said Shamima might be traveling under the name of 17-year-old Aklima Begum. Shamima is traveling with Kadiza Sultana, 16, and a third girl whose name police withheld at her family's request.
On Friday, police released pictures
of the three girls walking together through Gatwick Airport, carrying travel bags. Police say they are close friends who went to the same East London school.
"We are extremely concerned for the safety of these young girls and would urge anyone with information to come forward and speak to police. Our priority is the safe return of these girls to their families," Metropolitan Police Cmdr. Richard Walton said Friday.
"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."
UK police are concerned about the number of girls and young women trying to head to areas of Syria controlled by ISIS, Walton said.
"It is an extremely dangerous place, and we have seen reports of what life is like for them and how restricted their lives become," he said.