Heavy clashes between Assad regime forces and Daesh terrorist group in Deir el-Zour city have killed at least 73 fighters in the last 24 hours, a monitor said Sunday.

Assad regime controls most of Deir el-Zour city, capital of Deir el-Zour province in the country's east, and made further advances after responding to an Daesh attack that began Saturday, the Britain-based Syrian Observatory for Human Rights monitor said.

The monitor said the fierce fighting Saturday killed at least 50 Daesh fighters, as well as 23 Assad regime soldiers and pro-regime militiamen.

Observatory director Rami Abdel Rahman said Assad regime forces had captured two new neighborhoods and the municipal stadium.

"[Daesh] is now encircled in an area between the city and the (Euphrates) river," Abdel Rahman said.

Daesh once held large sections of Deir el-Zour city, and for nearly three years laid siege to other parts of it that remained under regime control.

In early September, advancing regime forces broke the siege, and they have been working since to expel the terrorists from the rest of the city.

Abdel Rahman said the fighting that began Saturday was the fiercest in the city since Assad troops broke the siege, adding that clashes were continuing Sunday, with regime ally Russia carrying out heavy air strikes in support of the army and allied fighters.

Deir el-Zour, an oil-rich province that borders Iraq, was once a stronghold of Daesh, but the terrorist group faces twin assaults there, from the regime and the U.S.-backed Syrian Democratic Forces (SDF), which is dominated by PKK terrorist group Syrian wing the Democratic Union Party (PYD) and its armed wing People's Protection Forces (YPG).

The terrorist have already been expelled from neighboring Raqqa province, and are now confined to just a few pockets of territory in Deir el-Zour.

More than 330,000 people have been killed in Syria since the conflict began in March 2011 with anti-Assad protests.