Police are "extremely concerned" about members of the missing Dawood family after contact was made with one of the missing women.
Detectives have received information that the family may already be in Syria.
Sisters Khadija Dawood, 30, Sugra Dawood, 34, and Zohra Dawood, 33, and their nine children, aged between three and 15, did not return to Bradford following a family trip to Saudi Arabia.
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West Yorkshire Police said: "We have received information that contact has been made with the family in the UK which suggests one of the missing adults may be in Syria.
"Contact has been made by one of the missing women and there is an indication that they may have already crossed the border into Syria but this is uncorroborated."
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.
"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.
"Anyone with information should call West Yorkshire Police on 101.
"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.
"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.
"Factually incorrect reporting could potentially jeopardise crucial lines of inquiry.
"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."
It has emerged that the sisters were questioned by security officials earlier this year while trying to fly to Saudi Arabia.
The sisters and their children missed a flight to Saudi Arabia in March after they were stopped and questioned by security staff at Manchester Airport.
It is believed they were originally booked to fly from Manchester to Jeddah for a religious pilgrimage on March 19.
However, they were stopped and questioned, causing them to miss the flight, which they later re-booked.
Counter-terrorism officials have confirmed the group had been stopped for questioning in March but were not prevented from travelling after enquiries revealed that they were heading on a pilgrimage.
The Leeds-based North East Counter Terrorism Unit said: "As a result, enquiries 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 enquiries caused the group to miss the flight
Husbands of two of the sisters begged their families to come home yesterday.
Akhtar Iqbal and Mohammed Shoaib broke down as they pleaded desperately for the group to return so they can resume their "normal lives".
The group went on an Islamic pilgrimage to Saudi Arabia but instead of returning were thought to have gone to Turkey, and possibly on to Syria.
West Yorkshire Police and Crime Commissioner Mark Burns-Williamson called for a review of the Government's counter-radicalisation Prevent strategy.
He told BBC Radio 4's Today programme that more work needed to be done to tackle "online grooming", adding: "I am certainly taking steps in developing something called Community Voices which is an online platform to help put a different narrative to what's happening.
"Clearly there is a different dimension to this one, with mothers of young children apparently or potentially taking them out to a war-torn area such as Syria."
Mr Burns-Williamson refused to comment on reports that the Dawoods were under surveillance, saying: "It's not for me at this point to get into the operational questions around this."
Following the disappearance of the Dawood sisters and the death of 17-year-old Talha Asmal, from Dewsbury, who is reported to have taken part in a suicide bombing in Iraq, Mr Burns-Williamson said: "In West Yorkshire we have had two very unfortunate incidents. What I do know is that there are some very strong communities in West Yorkshire and I am sure they will pull together.
"In terms of the Prevent strategy there is clearly a need to review that, to revisit it and to work with Government and other authorities to make sure that some of these very difficult discussions do take place in our communities and I will be seeking meetings with ministers to do that."
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