Police are "extremely concerned" about members of the missing Dawood family after receiving information that they may already be in Syria.
British sisters Khadija Dawood, 30, Sugra Dawood, 34, and Zohra Dawood, 33, and their nine children, aged between three and 15, are feared to have travelled to link up with terror group Islamic State in the country.
One of the sisters has made contact "and there is an indication that they may have already crossed the border into Syria", West Yorkshire Police said.
Their brother is believed to be fighting for Islamic State in Syria and he reportedly persuaded his sisters over Skype to join him.
Meanwhile, it emerged that the same group tried to fly earlier this year.
The BBC reported that the sisters and their children, from Bradford, were originally booked to travel from Manchester to Mecca and Medina on March 19.
The North East Counter Terrorism Unit confirmed that the same group had been stopped and made the subject of security checks earlier this year.
It said in a statement: "As a result, inquiries revealed that the visit was a religious pilgrimage to Saudi Arabia and the group were not prevented from travelling. However, the process of conducting those inquiries caused the group to miss the flight.
"On a daily basis a number of people are subject to security checks prior to boarding flights.
"It is worth repeating that the party were not legally prevented from travelling."
Today's developments came after two of the women's husbands broke down in tears yesterday as they pleaded with their wives to come home.
Assistant Chief Constable Russ Foster said: "We are extremely concerned for the safety of this family, especially the nine children aged between three to 15 years.
"The family have reached out to Khadija, Sugra and Zohra in the hope that they hear their messages, and we encourage them to make contact with us so we can return them and the children safely to their families, who are extremely worried about them."
He warned that Syria is "extremely dangerous" - a place where British families can become trapped and not return.
He said: "We are concerned about anyone who has or is intending to travel to the part of Syria that is controlled by the terrorist group calling themselves Islamic State.
"It is an extremely dangerous place and not a place where young children should be taken.
"The choice of returning home from Syria is often taken away from those under the control of Islamic State, leaving families in the UK devastated and with very few options to secure their loved ones' safe return."
During an emotional press conference yesterday, Akhtar Iqbal and Mohammed Shoaib broke down as they pleaded desperately for their wives to return so they can go back to their "normal lives".
Mr Iqbal's wife Sugra has gone away with his five children - Ismaeel, three, Mariya, five, Zaynab, eight, Ibrahim, 14, and 15 year-old Junaid Ahmed.
Appealing directly to his family, he said: "Please, please call me. It's been eight, nine days, you are out and we don't know where you are.
"I miss you, I love you. All of you, I love you a lot. I can't live without you.
"To my family, please, please call me (so) at least I know where you are. Are you safe?"
Mr Shoaib's wife Khadija has disappeared with his two children - five-year-old Muhammad Haseeb and Maryam Siddiqui, seven.
He said: "Please come back home with the kids, I know the kids can't live without me and you. Please bring them home, they can't live without me."
He added: "I'm not angry, please come back, everything is normal, come back to normal life, please."
The group travelled to Saudi Arabia on May 28 to go on an Islamic pilgrimage.
They were due to return to the UK on June 11, but broke off all contact with family in Britain two days earlier on June 9.
It is believed they boarded a flight to Turkey that day and may now have crossed into Syria.
The police urged anyone with information to get in touch.
Mr Foster said: "We would urge anyone with information about the family's whereabouts to come forward and speak to police so the family can safely return to the UK."
He added: "Police have been working with the extended Dawood family and the Turkish authorities since they were reported missing in an attempt to locate the family and return them to their loved ones.
"This has been an intensive investigation and we are committed to ensuring the correct information is shared."
He said anyone with information should call West Yorkshire Police on 101.
He added that "any piece of information, no matter how small, could help the UK or overseas authorities to locate the family so that they can be safely returned home to their loved ones".
Metropolitan Police Commissioner Sir Bernard Hogan-Howe said it was "almost impossible to believe" that the three missing sisters may have taken their children to the "war zone" that is Syria.
Speaking to BBC London 94.9, he said: "It's pretty significant. We've seen thousands of people go to Syria, and now we're seeing this last 12 months more younger people going, and now we're seeing children going often accompanied by women.
"It's a real worry - we're not seeing thousands going in that category (children) but certainly tens and obviously it looks like that's what's happened in West Yorkshire. We've got mums who've taken their children to a war zone, and it's almost impossible to believe, isn't it?
"So we are worried about how we influence those people, and stopping them doing it."