Russian fighter jets killed more than 200 fighters from the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) in a recent air attack in Syria, the defence ministry said on Monday, as part of an intensified air campaign to help President Bashar al-Assad's forces drive the group from its major stronghold of Deir Az Zor.

The planes destroyed an armed convoy of at least 20 ISIL vehicles that were heading to Deir Az Zor, the ministry said in comments carried by state news agency TASS. It did not specify the date of the attack.

The Syrian Observatory for Human Rights, a UK-based group monitoring the Syrian conflict via a network of contacts on the ground, confirmed that at least 70 fighters from ISIS were killed in a Russian air attack on a convoy in the countryside west of Deir Az Zor on Friday.

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This month alone, Russian fighter jets have flown 990 missions, destroying 40 armoured vehicles, more than 100 trucks and killing about 800 ISIL fighters, said Colonel General Sergei Rudskoi. Those casualties include more than 200 killed in the recent raid on the ISIL convoy.

Rudskoi said Syrian troops are meanwhile advancing from three directions to encircle Deir Az Zor city.

Syrian government forces control around half the city and a nearby airbase, both of which are besieged by ISIL fighters. Rudskoi said fighters fleeing US-backed offensives in the Iraqi city of Mosul and the Syrian city of Raqqa have poured into Deir Az Zor in recent months.

He said Syrian troops have also encircled Akerbat, an ISIL-held town in central Syria, and that Russian fighter jets have targeted ISIL convoys trying to flee the area for Deir Az Zor.

Russia has waged an air campaign in Syria since September 2015 aimed at helping Assad's forces defeat ISIL as well as Syrian rebel groups.

Rudskoi said the creation of de-escalation zones in Syria had helped the Russian military to focus on battling ISIL.

Under a plan approved in May by Russia, Turkey and Iran that applies to three areas of the country, the government and Syrian rebels have agreed to cease hostilities, halt government air raids and improve humanitarian access.

Russian military police are monitoring compliance at checkpoints around the three zones.