Russian President Vladimir Putin said Thursday that Islamic terrorists had detained several U.S. nationals south of the euphrates river in Syria.

Putin added that Islamic State militants had seized nearly 700 hostages in part of Syria controlled by U.S.-backed forces and issued an ultimatum promising to execute 10 people every day.

Speaking in the Black Sea resort city of Sochi, Putin said the hostages included several U.S. and European nationals, adding that Islamic State was expanding its control in territory controlled by U.S. and U.S.-backed forces.

Putin did not specify what the militants' demands were.

Open gallery view Locations of Islamic State operations in Iraq and Syria Credit: Reuters graphic

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The TASS news agency reported on Wednesday that ISIS militants had taken around 700 hostages in Syria's Deir-al Zor province after attacking a refugee camp in an area controlled by U.S.-backed forces on Oct.13.

TASS said the militants had kidnapped around 130 families and taken them to the city of Hajin.

The United States’ top military officer said on Tuesday that little progress had been made in dealing with the underlying conditions that have given rise to armed Islamist militants, even as military gains have been made against groups like Islamic State in Iraq and Syria.

“Little progress has been made in addressing the underlying conditions that lead to violent extremism,” said Marine General Joseph Dunford, chairman of the Joint Chiefs of Staff.

U.S. backed forces and Iraqi militias liberated nearly all of the territory that Islamic State, also known as ISIS, once controlled in Iraq and Syria.

“Challenges remain in our political, our military, our intelligence, and our law enforcement cooperation despite the fact that we’ve had some positive trends and cooperation; clearly there is much more to be done,” said Dunford, who was speaking during a conference countering violent extremism.

One of the issues facing U.S. backed forces in Syria is the large number of foreign fighters being detained.