Russia says ISIS is concentrating forces and heavy weaponry in the western city of Dayr al-Zawr to prevent its last stronghold in Syria from being captured by the Syrian government.

Russian Defense Ministry said Sunday that a recent joint push by the Syrian army and Russian air force near the town of Ghanem Ali in Raqqah Province killed some 800 ISIS terrorists.

Among the fatalities were the “most capable and best armed group of [ISIS] in the Euphrates River valley,” it said.

Earlier, Russia’s Defense Ministry published footage showing stages of the air raids in Ghanem Ali.

It said that following the successful offensive action of the Syrian Armed Forces group supported by the Russian Air Forces in the Ghanem Ali area, ISIS terrorists were trying “to regroup and establish fortified defensive positions in the Dayr al-Zawr area.”

The Russian Defense Ministry also said ISIS’s onrush toward the city showed that the terrorist group recognized the fact that “the staging area around Dayr al-Zawr is the last stronghold of the international terrorists on Syrian soil.”

Since the beginning of the Syrian civil war, terrorist groups have been fighting the Syrian government. In September 2015, Russia launched an aerial campaign in Syria in support of the government’s fight against terrorism.