ANKARA (Reuters) - Three British teenagers who were planning to join the Islamic State in Iraq and Syria (ISIS) group were detained in the Turkish city of Istanbul, Turkish officials said on Sunday.

The three, who have not been named, were detained on Friday, Turkish sources told Reuters. They said arrangements were being made with British authorities to deport them this week.

Police in London said they had been made aware that two 17-year-old boys had gone missing and were thought to be travelling to Syria. Further inquiries revealed they had travelled with a 19-year-old male.

"Officers alerted the Turkish authorities who were able to intercept all three males, preventing travel to Syria. They remain in detention in Turkey. The families have been kept informed of developments," the police statement said.

Three British schoolgirls entered Turkey last month and are thought to have joined ISIS in Syria.

On Thursday, the Turkish Foreign Minister said the girls had been helped to cross the border by a spy working for one of the countries in the US-led coalition against the militants.

ISIS controls swathes of territory in Syria and Iraq. It has attracted thousands of foreign fighters from Europe and elsewhere.

Turkey has faced criticism for not better controlling its southeastern borders, but has accused European countries of failing to prevent would-be extremists from leaving in the first place.