"The strikes hit a cattle market in the Tal Hamis area, which is controlled by IS," said the Britain-based Syrian Observatory for Human Rights, which has a wide network of sources inside Syria.
Previously it had said 27 people were killed in yesterday's raid.
Tal Hamis lies in Hasakeh province which is mainly controlled by Kurdish militia or regime troops, but where the jihadists hold some areas.
The pro-government newspaper Al-Watan said dozens of IS members were killed in the operation south of Tal Hamis.
IS controls a number of oil fields in eastern Syria and Washington says they provide the group with significant revenue.
The jihadists' black market oil infrastructure has been repeatedly targeted by US-led air strikes.