The Russian military said Wednesday it had bombed the Syrian province of Deir Ezzor after a deadly Islamic State group assault saw jihadists tighten their siege of the provincial capital.

"The Russian operation conducted military operations only in the provinces of Latakia and Deir Ezzor in light of unfavourable weather conditions and to avoid risks for the civilian population," Russian news agencies quoted military spokesman Igor Konashenkov as saying.

Konashenkov added that Russian warplanes had struck 57 targets in 16 combat sorties in these two provinces in the past day.

The military spokesman also said Russia had delivered 50 tonnes of humanitarian aid to the besieged city of Deir Ezzor on Friday.

More than 40 tonnes of humanitarian cargo were delivered this week to the blockaded eastern Syrian city, the defence ministry said Tuesday.

The Syrian Observatory for Human Rights, a Britain-based monitor, also reported heavy Russian strikes in Deir Ezzor near to where IS has launched a bloody offensive on the provincial capital.

It said at least 13 people were killed in strikes on three locations in Deir Ezzor, with two children among the dead.

It added that heavy clashes were continuing between regime and IS fighters.

The monitor said IS jihadists had killed at least eight members of the pro-regime forces on Wednesday, including six who had been captured the previous day. At least one was beheaded.

Another seven were killed in fighting with the extremists, the monitor said.

IS launched its offensive on Deir Ezzor city on Saturday, with dozens of its fighters carrying out suicide bomb attacks as they stormed government positions.

The jihadists now control 60 percent of the city, intensifying a siege that had already caused fear and hardship for the roughly 200,000 people still living there.

Around 70 percent of the city's remaining residents are women and children, according to the United Nations.

The Observatory said late on Tuesday that IS had released 270 of more than 400 civilians it had abducted during its initial assault on Deir Ezzor.

The Russian military said Tuesday its jets had struck 579 "terrorist targets" in 157 combat sorties in the Aleppo, Raqa, Latakia, Homs, Hama and Deir Ezzor provinces.

Russia began an aerial campaign in support of Syria's government on September 30.

The Observatory said Wednesday that over 3,000 people had been killed in the strikes so far, including 1,015 civilians.

It reported at least seven people, including three children, were killed on Wednesday in apparent Russian strikes in Idlib province, in northwest Syria, which is under rebel control.

Moscow has dismissed allegations of civilian deaths in its strikes as "absurd".