The families of three missing British teenaged girls, who reportedly crossed over into Syria via Turkey to join the self-proclaimed Islamic State of Iraq and al-Sham (ISIS) militants, have blamed the school, Metropolitan Police, and the local authority for failing to handle the case appropriately.

They also urged for the safe return of their children on Monday. "We, the families of missing schoolgirls, pray for the immediate safe return of our beloved daughters. We feel our loss more acutely on Mothers' Day as we look over their beds and see only the spaces left behind by them," the families said in joint statement.

Shamima Begum, 15, Amira Abase, 15, and Kadiza Sultana, 16, left Bethnal Green Academy in east London in early February for Turkey and then reportedly crossed into Syria to join the extremist group.

"As you can imagine, the last few weeks have been incredibly difficult. We would like to thank the public for their generosity of spirit and support in this most trying of times," the statement said.

"With respect to the disappearance of our children we have been disappointed by the handling of this matter by the school, Metropolitan police and the local authority, all of whom, we feel, failed to act appropriately and pass on vital information to us or indeed between each other.

"As parents, we expect the safeguarding of our children to be the top priority of schools and the local authority whilst our children are in their care. Had we been made aware of circumstances sooner, we ourselves could have taken measures to stop the girls from leaving the U.K.," the statement added.

Before they went missing, British police reportedly had handed over the three girls a letter dated February 2 that asked them to seek permission from their families about giving deposition to authorities concerning the case of another missing British girl, who too left the U.K. in 2014 to join ISIS.

The girls did not show that letter to their parents and after they went missing, the families complained and accused the police of negligence for not approaching them directly with the letter. The families claim that they could have taken steps to prevent their daughters from leaving the U.K. had they known about the case.

British police in turn accused the families of having knowledge about their daughters' friend at school who went missing last year to join ISIS.

Also on Monday, London police released three British male teenagers who had been held on suspicion of preparation of terrorist acts, who were recently deported from Turkey after allegedly trying to cross into Syria to join the ISIS extremist group.

Earlier, a senior Canadian official said on Thursday that Canada was aware of reports suggesting that a suspect was in Turkish custody who worked for the Canadian intelligence agency and was allegedly involved in the case of the three British missing girls.