At least 23 civilians have been killed in aerial bombardment carried out by Syrian government forces in rebel-held parts of the Eastern Ghouta district, according to the Syrian Observatory for Human Rights (SOHR).

Both the monitor and the Syrian Civil Defence, a group of volunteer first responders, said at least 18 people died in a double-tap air raid in the besieged town of Misraba on Wednesday night, while three others were killed in the Bett Sawa area, including women and children.

Attacks on Eastern Ghouta, which lies on the outskirts of Damascus, the capital, have been frequent in recent days and are believed to be part of the Syrian government's assault on rebel-held positions in the area.

Russian planes, providing air support to Syrian President Bashar al-Assad's forces, have intensified shelling of the neighbourhood and its surrounding districts, including Arbin city in southern Syria.

On Wednesday, at least two people were killed in attacks carried out by Russian warplanes, SOHR said.

According to the group, the death toll is expected to rise as many injured are currently in "critical" condition.

The SOHR said 70 people have been killed in Eastern Ghouta since December 31. Al Jazeera cannot independently confirm the veracity of the casualty figures.

Eastern Ghouta has been targeted despite being among a handful of so-called de-escalation zones in Syria, where military activity is prohibited under a ceasefire agreement endorsed by Turkey, Russia, and Iran earlier this year.

The area is one of the last rebel strongholds in the country and is home to some 400,000 people.

Rebels in Eastern Ghouta have managed to keep Syrian military forces at bay during years of war, but a four-year government siege of the district has led to a humanitarian crisis with severe shortages of food and medicine.

Last week, aid agencies including the International Committee of the Red Cross (ICRC), Syrian Red Crescent and the Syrian American Medical Society (SAMS) began evacuating critically ill Syrians from Eastern Ghouta.

The Assad government has allowed in some aid, but the UN says its current level of assistance covers just about 10 percent of the besieged population.

Last month, the UN said civilians who remain trapped there are facing a "complete catastrophe" because the government has blocked aid deliveries.

More than 600 people are in need of urgent medical care, according to SAMS.

Medicine is being rationed, and people are dying of complications due to the limited availability of simple procedures like dialysis.

Syria's conflict, which started with peaceful anti-government demonstrations in March 2011, escalated into a full-blown war that has claimed more than 300,000 lives and driven about half of the country's prewar population of 22 million from their homes.

Forces loyal to Assad and those opposed to his rule continue to battle each other, as well as fighters from the Islamic State of Iraq and the Levant (ISIL) group.