SOCHI, Russia (Reuters) – President Vladimir Putin said on Thursday that Islamic State (IS) militants had seized nearly 700 hostages in part of Syria controlled by U.S.-backed forces and had executed some of them and promised to kill more.

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 on the left bank of the River Euphrates controlled by U.S. and U.S.-backed forces.

Putin did not specify what the militants’ demands were.

“They have issued ultimatums, specific demands and warned that if these ultimatums are not met they will execute 10 people every day. The day before yesterday they executed 10 people,” Putin told the Valdai discussion forum in Sochi.

The TASS news agency reported on Wednesday that IS 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.

(Reporting by Gleb Stolyarov in Sochi; Additional reporting by Polina Devitt in Moscow; Writing by Tom Balmforth; Editing by Andrew Osborn)