In the message broadcast yesterday on Al-Bayan radio, IS said it was behind explosions targeting Kurdish new year festivities in northeast Syria and raids on government positions in the central province of Hama.
Forty-five people, including at least 12 children, were killed in twin bombings in the northeastern city of Hasakeh on Friday as they marked their new year's eve, the Syrian Observatory for Human Rights said.
Observatory head Rami Abdel Rahman said people were too scared to celebrate yesterday, fearing more attacks.
The attacks, which took place on the bloodiest day in Syria so far this year, were widely condemned.
"Committing these barbaric crimes on this day, which amounts to a national holiday in Syria, shows that the killers have no link to the Syrian people or to their values. In fact, they lie in wait for Syrians," said the Syrian National Council, the largest bloc in the opposition National Coalition.
"This description applies to the regime of (President Bashar) al-Assad and to the extremist terrorist organisations it has produced," it said.
IS declared a "caliphate" in June 2014 that straddles large parts of Iraq and Syria under its control.
In its radio statement, the Sunni Muslim extremist group also claimed responsibility for a "special operation" Friday on a government supply route linking the provinces of Hama and Aleppo to the north.
"The soldiers of the caliphate stormed regime checkpoints," Al-Bayan said, killing more than 100 regime soldiers and 15 fighters from the Lebanese militia group Hezbollah, which has intervened in Syria on behalf of the government.
