Seventeen civilians including eight children were killed in air strikes on a market in eastern Syria on Monday, the first day of the holy Muslim month of Ramadan, a monitor said.
The Syrian Observatory for Human Rights said the strikes on Al-Shaara, a town held by the Islamic State (IS) group in Deir Ezzor province, were suspected to have been carried out by either Russian or Syrian government planes.
The death toll is likely to rise due to the serious condition of some of the wounded, Observatory chief Rami Abdel Rahman said.
Residents from nearby villages typically flock to the Al-Shaara market to do their shopping, Abdel Rahman said.
Heavy air strikes hit IS group -held areas in and outside the divided city of Deir Ezzor from early Monday, he said.
IS controls more than 60 percent of Deir Ezzor city, the capital of the oil-rich province of the same name, most of which has been in its hands since 2013.
The Observatory relies on a network of sources inside Syria to gather its information on the five-year-old conflict, which has killed more than 280,000 people and displaced millions.
It says it determines whether strikes were carried out by Syrian, Russian or US-led coalition aircraft based on the location of the raids, flight patterns and the types of planes and munitions involved.
Regime air strikes killed at least 15 civilians in the IS-held area of Boleel outside Deir Ezzor city on Friday, according to the monitor.