Russian President Vladimir Putin attends a signing ceremony following a meeting with Egyptian President Abdel Fattah al-Sisi in the Black Sea resort of Sochi, Russia October 17, 2018. Pavel Golovkin/Pool via REUTERS

SOCHI, Russia (Reuters) - President Vladimir Putin said on Thursday that Islamic State 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 Islamic State 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.

In Washington, the U.S. military cast doubt on Putin’s claims.

“While we have confirmed that there was an attack on an IDP (internally displaced persons) camp near (Deir-al Zor) last week, we have no information supporting the large number of hostages alleged by President Putin and we are skeptical of its accuracy,” Commander Sean Robertson, a Pentagon spokesman, said in a statement.

“We are also unaware of any U.S. nationals located in that camp,” Robertson added.