ANKARA (REUTERS) - A suspected associate of British Islamic State in Iraq and Syria (ISIS) leader "Jihadi John" is being held by authorities in Turkey, two senior Turkish officials said on Friday, a day after the United States targeted the militant in an air strike in northern Syria.

A man thought to be Aine Lesley Davis, one of a group of British Islamists believed to have been assigned to guard foreign prisoners in Syria, was detained in Istanbul, the officials said.

They declined to give further details, saying investigations by the police and intelligence agencies were continuing.

British Prime Minister David Cameron said he could not yet confirm the death of Mohammed Emwazi, who was dubbed Jihadi John after appearing in videos showing the killings of US and British hostages, and the Pentagon said it was still assessing the effectiveness of Thursday's strike.

But a US official said the attack in the northern Syrian town of Raqqa, Islamic State's de facto capital, had probably killed Emwazi.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, quoted sources in Raqqa as saying Emwazi had been blown to pieces.

Former hostages of ISIS have suggested the group of British Islamists was assigned to guard foreign prisoners. In the British media they were nicknamed John, Paul and Ringo, after members of the Beatles, because of their English accents.

Emwazi took part in videos showing the murders of US, British and other hostages. Dressed entirely in black, a balaclava covering all but his eyes and the bridge of his nose, he became a menacing symbol of Islamic State brutality and one of the world's most wanted men.