London (AFP) - The families of three schoolgirls who fled their London homes for the conflict in Syria have voiced anger at the police, saying they did not warn them that the teenagers risked being radicalised.

School friends Kadiza Sultana, 16, and 15-year-olds Shamima Begum and Amira Abase left their homes and flew to Istanbul, from where they are believed to have joined Islamic State (IS) jihadists in Syria.

All three girls, plus four others, were spoken to in December, and again in February, at the school by counter-terror police investigating the disappearance of one of their close friends -- referred to as Girl 1 -- who went to Syria.

But the three girls hid a courtesy letter that police had given them to pass on to their parents after the February meeting, in which they requested further permission to speak to the teenagers.

The relatives say police should have given the letter directly to the families, which might have alerted them to any danger.

Amira's father Abase Hussein told ITV television Friday that he could have prevented his daughter from travelling if he had received the letter.

"We would have stopped them," he said. "We would have discussed it and taken away their passports from them. This wouldn't have happened."

Kadiza's sister Halima Khanom added: "We wouldn't have been here today doing this if we'd got that letter and known what was going on."

Scotland Yard police headquarters insists there was no indication at the time that the girls would follow their friend to Syria, but admitted they should have communicated more directly with the families.

"With the benefit of hindsight, we acknowledge that the letters could have been delivered direct to the parents," it said Saturday.

"However, the parents were already aware from the deputy head that Girl 1 had travelled to Syria," the police said in a statement.

"The teenagers were all being co-operative, they were all being treated as potential witnesses and there was nothing whatsoever to indicate that they themselves were planning to travel to Syria," the statement said.

"It remains a priority to try to prevent people travelling to join terrorist groups whether they be determined terrorists, radicalised individuals or vulnerable teenagers."