At least 34 civilians, including nine children, have been killed in a Russian air raid on Euphrates River ferries near Syria's Deir Az Zor city, according to a monitoring group.

Rami Abdel Rahman, head of the UK-based Syrian Observatory for Human Rights, said the air raids on Sunday targeted "more than 40 ferries" that left al-Boulil town southwest of Deir Az Zor for the eastern shore of the Euphrates.

No immediate response from Russian authorities was available.

Sunday's attack came as Syrian troops pressed an offensive against the Islamic State of Iraq and the Levant group (ISIL) across Deir Az Zor province with Russian air cover.

According to the War Media channel operated by Syrian government ally, Hezbollah, Syrian troops seized full control of the 450km road linking the capital, Damascus, to Deir Az Zor for the first time in four years.

"The Syrian army and its allies now control the entire international highway between Deir Az Zor and Damascus, through the cities of al-Sukhna and Palmyra," it said, referring to two other central Syrian cities recaptured from ISIL (also known as ISIS).

READ MORE: SDF in race with Syrian army for oil-rich Deir Az Zor

An alliance of Syrian Kurdish and Arab fighters backed by the United States was also on the move against ISIL in Deir Az Zor.

Syrian Democratic Forces (SDF) on Saturday announced it had begun clearing ISIL fighters from areas east of the Euphrates, which cuts diagonally across the province.

Sweeping advance

On Sunday, the Observatory said the SDF had made a sweeping advance, capturing territory just kilometres from Deir Az Zor city.

"They seized control of a hilltop seven kilometres from the eastern banks of the Euphrates," said Abdel Rahman.

He attributed the quick gains to the fact that "eastern parts of Deir Az Zor are desert and not densely populated".

Oil-rich Deir Az Zor province borders Iraq and is a strategic prize for both SDF and the Syrian army.

Since 2014, ISIL has held about 60 percent of the city and much of the surrounding province.