Speaking at a conference in Sochi on Thursday, Putin said ISIS seized almost 700 hostages in parts of Syria controlled by USA -backed forces.
Putin said this on Thursday while speaking at an global policy forum in Sochi, a city in the country.
Putin did not specify what the militants' demands were.
"They killed 10 people the day before yesterday", he said.
The state-run TASS news agency said the hostages were taken on 13 October after IS attacked a refugee camp in Deir az-Zour province in Syria's east.
"As a result, they took hostages and transported about 130 Arab families to [the city of] Hajin, which is under their control", the source said, according to TASS.
"ISIS attacked a refugee camp while the Syrian Democratic Forces tried to launch an offensive with support from the US-led coalition".
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His blunt talk Thursday comes as Russia-West relations remain frosty over the Ukrainian crisis, the war in Syria and the allegations of Russian meddling in the 2016 USA presidential vote.
And Putin's claims of the hostage situation have not been confirmed by anyone else so far, other than reporting an attack had, in fact, taken place.
In Washington, the U.S. military cast doubt on Putin's claims, the Reuters news agency reported.
Speaking Thursday at an worldwide policy forum, Putin said: "Those who believe that there was a murder must present evidence".
"We are also unaware of any United States nationals located in that camp", he added.
Nearly 3,000 Russian troops are now based in Syria.
Putin cautioned that Russia's new "hypersonic missiles" have given it a competitive edge over other countries, whereas he said other countries are one-and-a-half to 2 years behind the curve.
The weapons were given after it was reported Assad aims to increase his regional allies.