At least 77 civilians have been killed near Damascus in less than 24 hours of heavy bombardment by Syrian government forces as they prepare a ground operation to recapture the opposition-held enclave, a war monitor group said.

Air raids and artillery fired on Eastern Ghouta - a suburb of the Syrian capital - have killed at least 20 children since Sunday, the UK-based Syrian Observatory for Human Rights (SOHR) reported.

Some 300 people have been wounded during the attacks, according to the monitor.

"The heavy shelling targeted mainly all residential areas in Eastern Ghouta," Rami Abdel Rahman, head of the SOHR, told dpa news agency.

Government forces are preparing to launch a major ground assault on Eastern Ghouta following the heavy bombardment, which began on Sunday, SOHR said.

Government planes are "shooting everything that moves inside the residential areas", a local doctor told dpa.

"Our hospitals are overcrowded with wounded. We are running out of anaesthetics and other essential medications," he said.

Activists put the number of dead at 68. "Each minute between 20-30 shells are falling on the residential areas, especially in Hammouriyeh and Sabka," Mazen al-Shami was quoted as saying from the enclave.

The main opposition National Coalition, which is based in Turkey, denounced the "war of extermination" in Eastern Ghouta as well as the "international silence".

In a statement, it also accused Syria's ally, Russia, of seeking to "bury the political process" for a solution to the conflict.

Eastern Ghouta, the last remaining rebel-held area near Damascus, has been under siege by government forces since 2013. It is home to about 400,000 people.

An international aid convoy was able to deliver much-needed food and medical supplies to the enclave on February 14.

#OnTheGround in #Syria. Finally a cross line convoy into #EasternGhouta with enough @WFP provided food for over 7000 civilians for a month. We need much more such convoys. Fighting has to stop to deliver much needed aid to all civilians in need. https://t.co/VyVouG2LlG — Jakob Kern (@JakobKern1961) February 14, 2018

However, the convoy's deliveries reached only 2.6 percent of the estimated 272,500 people in need of humanitarian aid, according to Ali al-Za'tari, the UN's humanitarian coordinator in Syria.

The Syrian government and its allies - including Russia, Iran and Turkey - classified Eastern Ghouta as a de-escalation zone in an apparent bid to lessen fighting in the area last year.

Syria's other de-escalation zones include parts of the northeastern province of Idlib, areas in northern Homs province, and rebel-controlled territory in the south near the border with Jordan.

Despite the agreement, nearly 200 people were killed in Eastern Ghouta and in Idlib within a four-day period earlier this month.

Hundreds of thousands of people have died in fighting during Syria's seven-year civil war, and millions have fled the country.