The death toll from a recent wave of attacks by Russian and Syrian warplanes on areas controlled by the Islamic State (IS) has risen to 164 over the past 72 hours, according to the Syrian Observatory for Human Rights (SOHR). The dead are said to include 43 children and 25 women.

The UK-based monitoring group said the attacks targeted towns across the countryside in the eastern province of Deir al-Zor, and Raqqa, the de facto capital of IS's self-proclaimed caliphate. The SOHR described the bombings as "revenge massacres." Last week, IS reportedly kidnapped an estimated 400 civilians from Syrian government-held areas in Deir al-Zor. Though 270 of the captives were later released, the militants were accused of carrying out beheadings and targeting pro-government fighters and their families.

The SOHR said 63 people, including nine children, were killed in Khesham, a town in the eastern countryside of Deir al-Zor. Another 55 people, including 18 children and nine women, were reportedly killed in al-Bolel, al-Kharita, and other villages in the western countryside of of the province. Sixteen children and 13 women were among the 44 people killed in Raqqa, the group said.

IS controls most of Deir al-Zor, a strategically important area that links Raqqa with Mosul, the largest city under the group's control in northern Iraq. The remaining areas under control of forces loyal to Syrian President Bashar al-Assad have been under siege since last March, and residents are said to face severe food shortages and grim conditions.

Russia intervened in the Syrian conflict last September, and the added air power has reportedly turned the military balance in the war in favor of the Assad regime, which is battling both IS and an array of allied rebel groups. In December, Amnesty International released a report that said Russian airstrikes have indiscriminately killed hundreds of civilians and may constitute a war crime.

The human rights organization interviewed dozens of witnesses and activists, and reviewed video clips and images of six Russian attacks and their aftermath, disputing the Kremlin's claims that it has only attacked "terrorist" targets. The group indicated "serious failings" by Russia to adhere to international law.

"The Russian armed forces appear to have directly attacked civilians or civilian objects by striking residential areas with no evident military objective and even medical facilities, resulting in deaths and injuries to civilians," Amnesty said in its report. "In others, they seem to have attacked military objectives and civilian objects without distinction, or caused disproportionate harm to civilians when striking military targets. Such attacks may constitute war crimes."