Renu Begun, sister of teenage British girl Shamima Begun, holds a photograph of her sister as she makes an appeal for her to return home at Scotland Yard, in London, on February 22, 2015. Shamima Begun is one of three teens who left their London homes on Tuesday and travelled to Istanbul without telling their families. Photo: Laura Lean

London - British police said on Tuesday they believed that three London schoolgirls who travelled to Turkey and are thought to be trying to join militant Sunni Islamist group Islamic State, have now crossed into Syria.

Friends Amira Abase, 15, Shamima Begum, 15, and Kadiza Sultana, 16, flew to Istanbul from London on February 17.

“Officers leading the investigation into the three missing schoolgirls from east London, now have reason to believe that they are no longer in Turkey and have crossed into Syria,” police said in a statement.

They gave no further details, but said they continued to work closely with Turkish authorities on the investigation.

The girls' plight has prompted widespread concern in Britain, with Prime Minister David Cameron urging social media firms to do more to deal with online extremism, saying the girls appeared to have been radicalised “in their bedrooms”.

He also said airlines needed new systems to vet children travelling alone.

The three are friends with a fourth teenage girl from the same school who police believe is already in Syria, having travelled to Turkey in December.

Police said their families, who have issued urgent appeals for their daughters to return, have been surprised and devastated by their disappearance.

Security forces estimate some 600 British Muslims have travelled to the region to join the conflict there, some of them with Islamic State.

Around half have since returned, and dozens have been arrested in Britain under anti-terrorism legislation.